The Complete Guide to VoIP for Multi-Site UK Businesses

Managing phone systems across multiple UK locations used to require separate PBX boxes, separate contracts, and separate phone numbers at each site. In 2026, a single cloud VoIP system connects every office under one platform.

Here is how to do it properly — from a consultant who has connected 80+ multi-site UK businesses.

The Architecture

Cloud VoIP Platform (single system)
    |
    +--- London HQ (50 users)
    |    Extensions: 1000-1049
    |    Local number: 020 XXXX XXXX
    |
    +--- Manchester (25 users)
    |    Extensions: 2000-2024
    |    Local number: 0161 XXX XXXX
    |
    +--- Edinburgh (15 users)
    |    Extensions: 3000-3014
    |    Local number: 0131 XXX XXXX
    |
    +--- Birmingham (20 users)
         Extensions: 4000-4019
         Local number: 0121 XXX XXXX

Every office has local phone numbers. Every employee has the same extension regardless of location. Internal calls between offices are free and instant — dial the 4-digit extension.

What Multi-Site VoIP Solves

Problem (Old Way)	Solution (Cloud VoIP)
Each office has separate phone system	One system, one admin portal
Inter-office calls cost money	Free — internal extensions
Cannot transfer calls between sites	Transfer to any extension, any site
Each office has separate auto-attendant	Unified IVR with site-specific routing
New office needs PBX installed	Add users in admin portal, ship phones
Reception cannot see who is free at other sites	Presence indicators across all sites

The Setup Process for Each New Site

Step	Time	What Happens
1. Assign extensions	5 min	Add users in admin portal
2. Get local number	Instant	Choose 01/02 area code number
3. Ship phones	Next day	Pre-configured, auto-provision on arrival
4. Configure routing	15 min	Ring groups, auto-attendant for new site
5. Port existing numbers	5-10 days	If keeping old numbers

Total effort to add a new site: 30 minutes of admin time + wait for phones to arrive.

Compare to the old way: order PRI circuits (4-6 weeks), buy PBX hardware (procurement + delivery), hire installer (schedule + travel), configure (2-3 days on-site).

Network Requirements Per Site

Site Size	Concurrent Calls	Bandwidth Needed	Recommended Connection
5-10 users	3-5	500 Kbps	Any broadband
10-25 users	8-12	1.2 Mbps	Business broadband
25-50 users	15-25	2.5 Mbps	Business fibre
50-100 users	30-50	5 Mbps	Dedicated fibre

Critical at every site:

• QoS configured on the local router
• SIP ALG disabled
• Voice VLAN if possible (not mandatory for < 25 users)

Call Routing Strategies

Strategy 1: Follow-the-Sun

Calls route to the office that is currently in business hours:

• 8 AM - 5 PM UK: London/Manchester handle calls
• 5 PM - 8 PM UK: If you have a team in a later timezone, route there
• After hours: voicemail or on-call mobile

Strategy 2: Overflow Between Sites

If London reception is busy (all agents on calls), overflow to Manchester reception automatically. The caller does not know they are talking to a different office.

Strategy 3: Skills-Based Across Sites

Billing queries go to the billing team — regardless of which office they sit in. A billing-trained agent in Edinburgh takes the call if London billing is busy.

Common Multi-Site Mistakes

Mistake	Impact	Fix
Different VoIP providers per site	Cannot transfer between sites	Use one provider for all sites
No QoS at remote sites	Poor quality at smaller offices	Configure QoS everywhere
No backup internet at critical sites	Office goes dark if ISP fails	Dual ISP or 4G backup
Extension numbering conflicts	Extension 100 exists at 3 sites	Use site-prefix numbering (1xxx, 2xxx)

Cost Comparison

Approach	4-Site Monthly Cost	Setup Cost
Separate PBX per site	£6,800	£45,000
Hosted PBX (separate instances)	£3,400	£5,000
Unified cloud VoIP	£2,640	£0

Unified cloud VoIP is 61% cheaper than separate PBXes and requires zero upfront investment.

DialPhone specialises in multi-site UK deployments. One system, one bill, one support team across all your locations. Each site gets a local number, instant inter-site calling, and unified management from a single admin portal.