Some finance directors want proof, not promises. Fair enough. Here is the mathematical model for VoIP cost savings, with every variable exposed and every assumption documented.
The Model
Variables
N = number of phone users
Cl = current monthly cost per line (legacy)
Cv = VoIP monthly cost per user
H = hardware cost per desk phone (one-time)
P = percentage of users needing desk phones (vs softphone-only)
I = implementation cost (one-time)
M = months in analysis period
Legacy Total Cost of Ownership (M months)
TCO_legacy = N × Cl × M
Note: legacy Cl includes line rental, maintenance, features, long distance, and IT admin time allocated per user.
VoIP Total Cost of Ownership (M months)
TCO_voip = (N × Cv × M) + (N × P × H) + I
Monthly Savings
S_monthly = N × (Cl - Cv)
Break-Even Point (months)
BEP = (N × P × H + I) / (N × (Cl - Cv))
Return on Investment (over M months)
ROI = ((N × (Cl - Cv) × M) - (N × P × H + I)) / (N × P × H + I) × 100%
Plugging In Real UK Numbers
Typical UK Values
| Variable | Legacy Value | VoIP Value | Source |
|---|---|---|---|
| Cl (per user/month) | £58 | — | Avg from 500 bill audits |
| Cv (per user/month) | — | £24 | DialPhone standard plan |
| H (per desk phone) | — | £130 | Yealink T33G |
| P (% needing desk phone) | — | 40% | Industry average 2026 |
| I (implementation) | — | £0 | DialPhone includes it |
Results by Company Size
| Users (N) | Legacy/month | VoIP/month | Monthly Save | Hardware | Break-Even | 3-Year Savings | 3-Year ROI |
|---|---|---|---|---|---|---|---|
| 10 | £580 | £240 | £340 | £520 | 1.5 mo | £11,720 | 2,154% |
| 25 | £1,450 | £600 | £850 | £1,300 | 1.5 mo | £29,300 | 2,154% |
| 50 | £2,900 | £1,200 | £1,700 | £2,600 | 1.5 mo | £58,600 | 2,154% |
| 100 | £5,800 | £2,400 | £3,400 | £5,200 | 1.5 mo | £117,200 | 2,154% |
| 200 | £11,600 | £4,800 | £6,800 | £10,400 | 1.5 mo | £234,400 | 2,154% |
Sensitivity Analysis
What if our assumptions are wrong?
| Scenario | Cl | Cv | P | Break-Even | 3-Year Savings (50 users) |
|---|---|---|---|---|---|
| Base case | £58 | £24 | 40% | 1.5 mo | £58,600 |
| Legacy cheaper than avg | £42 | £24 | 40% | 2.4 mo | £29,000 |
| VoIP more expensive | £58 | £32 | 40% | 1.7 mo | £44,200 |
| Everyone needs desk phone | £58 | £24 | 100% | 3.8 mo | £55,000 |
| High implementation cost | £58 | £24 | 40% | 1.8 mo | £56,100 |
| Worst case (all above) | £42 | £32 | 100% | 13 mo | £11,800 |
Even in the absolute worst case — where legacy is cheaper than average, VoIP is more expensive than expected, every user needs a desk phone, and implementation costs extra — VoIP still saves £11,800 over 3 years with a 13-month break-even.
In the base case, break-even is 6 weeks and 3-year savings are £58,600.
The Proof
VoIP saves money under EVERY realistic scenario because:
Cl > Cv in 100% of the 500 bills we audited. The average legacy per-user cost (£58) is 2.4× the average VoIP per-user cost (£24).
Hardware cost is one-time. A £130 phone is paid once. The £34/month saving recurs every month for years.
60% of users do not need desk phones. Softphone + headset (£60) costs less than a desk phone (£130).
Implementation cost is £0 with most cloud providers. Even if it is £5,000, it is recovered in 3 months of savings.
The maths does not require faith. It requires a calculator.
For your own calculation, use the model above with your actual numbers. Or send your invoices to DialPhone for a free analysis — we will run the model for you and show you the exact break-even point for your business.
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