Super Funicular

Posted on Oct 10

Should You Leave the SIM Card in an Old Phone Security Camera? A Dual-SIM Guide for Offline, Wi-Fi and Mobile Data Setups

#android #privacy #security #mobile

Most old Android phones that become security cameras still have a SIM card inside. Often it is the old number from before the upgrade. Sometimes it is a cheap prepaid SIM that came with the phone. On a dual-SIM handset there may be two.

Nobody decides to leave them there. They are just there. And those SIM slots change how the camera behaves: whether it spends money on data, whether it comes back after a power cut, and whether your bank's one-time passwords show up on a phone sitting on a shop shelf.

This guide is for one question: which SIM, if any, should stay in the phone you use as a camera? It is written for the common case: a budget or older Android phone, a home or small shop, data that is prepaid and not unlimited, and no wish to pay a monthly fee for a camera.

Do I need a SIM card to use an old phone as a security camera?

No. A phone with no SIM at all still joins Wi-Fi normally. If the camera app records to the phone's own storage and serves its live view on your local network, the SIM does nothing for either job.

That is how Background Camera RemoteStream works. It records with the screen off to the phone's storage. A small web server on the phone itself serves the live view to any browser on the same Wi-Fi. No account, no cloud, no subscription. You only need the internet, through Wi-Fi or a SIM, if you want to push a YouTube Live stream that can be watched from outside the building.

So the real question is not "does the camera need a SIM?" It is "what does each SIM do while it sits in the camera phone?"

What a SIM can do to a camera phone (that you did not ask for)

Four things, in rough order of how often they cause trouble.

1. It can spend mobile data. If mobile data is switched on, Android can use it when the Wi-Fi is weak, has no internet, or is a shared network that needs a login. We explained why in why your phone security camera uses mobile data on shared Wi-Fi. With no SIM, or with mobile data off, that cannot happen.

2. It can lock itself after a restart. If the SIM has a SIM PIN turned on, Google's help page is direct: "You'll be asked for your SIM PIN whenever you turn on your device." After a power cut drains the battery and the phone restarts, that SIM has no mobile network until someone types the PIN by hand. If your camera depends on that SIM for internet, it stays offline until you visit it.

3. It receives calls and messages. A phone on a shelf still rings. It still lights up for SMS. And if the SIM is your main number, those messages include bank and app one-time passwords (OTPs), shown on a screen you are not holding.

4. It may be quietly deactivated. Prepaid SIMs that are never used do not last forever. In India, TRAI's 2013 rule says a prepaid connection "shall not be deactivated for any period of non-usage less than 90 days". That is a minimum, not a promise of more. Rules differ in Bangladesh, Iraq, Algeria and elsewhere, so check with your operator. A data SIM you set up once and forgot can stop working months later.

None of this is a fault in the camera app. It is the phone still being a phone.

Which SIM should stay in the phone? Four common setups

Setup 1: Home or shop with Wi-Fi, you watch on the same Wi-Fi

Take the SIM out. You lose nothing. Recording and the local live view both run without the internet. This is the true offline security camera: no SIM, no cloud, and nothing that can spend money.

If you cannot remove it (a tight tray, a glued back), turn mobile data off for that SIM instead, and turn off any option that switches to mobile data when the Wi-Fi is poor.

Setup 2: No router at all, using another phone's hotspot

The camera phone still needs no SIM for local viewing. Two phones on the same hotspot talk to each other directly; the live view is local traffic and does not need the internet, so mobile data can stay off. Either phone can make the hotspot. If the camera phone is the one making it and refuses to turn the hotspot on with no SIM inside (some phones behave this way), let the viewing phone make the hotspot instead and have the camera phone join it. The full setup, and what to check, is in how to use an old phone as a security camera without a Wi-Fi router.

Setup 3: You want to watch from outside, over mobile data

This is the one setup where a SIM in the camera phone earns its place. Keep one data SIM and set it up on purpose:

Choose it as the data SIM. Google's Pixel help says: "Only one SIM can be the default for data." On a Pixel the setting is under Settings > Network & internet > SIMs > Mobile data. Samsung, Xiaomi, Realme and others put it in different menus. Look for "SIM manager", "Dual SIM" or "Mobile data SIM".
Set a data limit. On a Pixel it is under Settings > Network & internet > SIMs > Data warning & limit. When the limit is reached, mobile data turns off and you get a notification. Google also warns that "some carriers measure usage differently," so set the limit below your real plan, not equal to it.
Turn the SIM PIN off on this SIM, or accept that every restart needs a visit. A camera that cannot come back online by itself after a power cut is a camera you will find dead.

There is one more thing to know before you pay for data. On mobile data, people outside usually cannot connect in to the phone. One reason is that an operator can put many customers behind one shared address, using its own network address translation. There is even an address range set aside for that kind of sharing (100.64.0.0/10, in RFC 6598). Whether your operator does this, and how, is up to your operator. The same RFC notes that some applications "cannot seed content due to the inability to open incoming ports". So the local web viewer that works on your Wi-Fi does not normally become reachable from across town just because there is a SIM inside.

What does work over mobile data is a connection the phone makes outward, like pushing a YouTube Live stream. That is why Setup 3 usually means YouTube Live.

And that uses real data. YouTube's own encoder guidance gives a minimum and a recommended bitrate for each quality. Converted to data per hour:

Quality (H.264)	YouTube minimum	Data per hour at minimum	YouTube recommended	Data per hour at recommended
480p, 30 fps	0.4 Mbps	about 180 MB	4 Mbps	about 1.8 GB
720p, 30 fps	3 Mbps	about 1.35 GB	8 Mbps	about 3.6 GB

A 720p stream at YouTube's minimum, left on for a full day, is about 32 GB. On most prepaid plans that is not a 24/7 setup. Stream when you need it, at a low quality, and let the phone record locally the rest of the time. The figures above are calculated from YouTube's numbers. Your own phone's data usage screen is the real measure, and we wrote about measuring it yourself rather than trusting anyone's estimate.

Setup 4: The SIM holds a number you still need for OTPs

Do not put it in the camera phone. Move it to the phone you carry. A camera phone sits in one place, often where customers or visitors can reach it. If it is picked up, your OTPs go with it.

If you truly have no choice, at least stop the lock screen from showing message text. On a Pixel: Settings > Notifications > Notifications on lock screen, then turn off Show sensitive content. Other brands have a similar "hide content" option under lock screen or notification settings. Also set the phone to silent, so it does not ring in an empty room at night.

Dual-SIM phones: what to do with the second slot

Many budget dual-SIM phones have two physical slots. The usual mistake is to fill both "because they are there". For a camera phone:

Empty is best unless you are using Setup 3.
One data SIM, chosen on purpose, if you are streaming out. Leave the second slot empty so nothing else can receive calls, messages or data.
Never your main number, for the reasons in Setup 4.

Newer phones add options such as using mobile data from both SIMs during calls, or switching data between SIMs automatically. These depend on your phone model and your operator. On a camera phone, turn off anything that lets data move to a SIM you did not choose.

A five-minute check after you set it up

Restart the phone on purpose. Does it come back to the network without you typing anything? If a SIM PIN prompt appears, you have found your power-cut problem today instead of later.
Unlock it once and open the camera app. If you use a screen lock, Android does not run most apps after a restart until the phone is unlocked once. Its developer documentation says: "By default, apps don't run during Direct Boot mode." Plan to unlock the phone after any restart, and check that recording has started again.
Open the live view from another device on the same Wi-Fi. That confirms the local path, which needs no SIM.
Check mobile data usage the next day. If the camera phone has no SIM, it should show nothing. If it has a data SIM, compare the number with what you expected from the table above.
Send the camera phone's number a test SMS (if it has a SIM) and look at the lock screen. If you can read the message without unlocking, so can anyone at the counter.

What the documentation does not tell you

Google's help pages describe each of these settings on a Pixel. They do not describe the same menus on Samsung, Xiaomi, Realme, Tecno or Infinix phones, which are most of the phones in this situation, so we have not printed paths for those brands. We have also not tried to say how long any particular operator keeps an unused SIM active outside India. And whether data from both SIMs can switch automatically depends on the phone and the operator, not only on Android. Test on your own phone before you rely on it.

If your stream drops but recording continues, that is a different problem with its own fixes, covered in live view stopped, recording did not.

The short version

Watching on the same Wi-Fi? No SIM.
Using a hotspot? No SIM in the camera phone.
Streaming out over mobile data? One data SIM, no SIM PIN, a data limit below your plan.
Your main number? Never in the camera phone.

Background Camera RemoteStream records with the screen off to the phone's own storage, serves a live view from a web server running on the phone itself, and can push to YouTube Live when you want a stream that reaches outside the building. No account, no cloud storage, no subscription. It is free on Google Play, and there is more at superfunicular.com.

Sources

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