Case Study: Microsoft Teams Calling | How a UK Professional Services Firm Replaced Its Phone System

This is a real engagement summary for a firm already on business VoIP thinking from a Microsoft 365 footprint. The figures below are this client's actual numbers, preserved as recorded.

What was the challenge?

The firm had run a Mitel on-premise PBX for seven years. The hardware was nearing end of support, maintenance costs rose year-on-year, and the system had no mobile working capability — a real gap for a team that worked from client sites and home offices. They needed a phone system that matched how they already worked.

The firm was already a Microsoft 365 Business Premium subscriber, and Teams was in daily use for internal meetings and collaboration. The open question for the IT team: could Teams Phone replace the Mitel PBX outright, or was a separate hosted phone system needed?

• Average monthly outbound call minutes: approximately 4,200 across the business
• Inbound: main number with receptionist routing, plus 6 direct dial numbers for senior staff
• Remote working: 70% of staff working from home or client sites at least 2 days per week
• Numbers to retain: 8 DDI numbers plus the main company number
• Compliance: call recording required for specific regulated teams

What did AMVIA do?

AMVIA ran a requirements assessment across call volumes, features, number retention and working patterns, then concluded Teams Phone with Operator Connect was the right fit. Because the firm already paid for Microsoft 365, Teams Phone licences were incremental rather than a whole new platform cost, and heavy existing Teams use meant minimal retraining. Delivery used VoIP built on Microsoft 365.

According to Microsoft Learn, Operator Connect lets a Microsoft-certified carrier connect its PSTN network directly to Teams through Microsoft's cloud — an alternative to Microsoft Calling Plans or Teams Direct Routing.

The rollout ran in five phases:

1. Licensing and carrier selection. Teams Phone add-on licences for all 35 users. At 4,200 minutes a month, Operator Connect per-minute pricing beat Microsoft's bundled Calling Plan tiers, so a UK carrier with a strong network SLA was selected. 2. Number porting. All 9 numbers (main plus 8 DDI) ported from the legacy Mitel/PSTN provider over 8 working days, cutting over simultaneously on a pre-agreed date with brief parallel running before the PBX was decommissioned. 3. Auto-attendant and call queues. A Teams auto-attendant handled business-hours routing to a receptionist queue, with after-hours voicemail and a separate out-of-hours message. Six staff got direct dial numbers; two queues served the busiest client-facing teams. 4. Compliance call recording. A Microsoft-certified compliance recording partner was deployed for the two regulated teams (6 users), recording all calls with tamper-evident storage and portal access. The other 29 users operate without recording. 5. Device transition. Endpoints matched working patterns rather than a one-size rollout.

Hardware cost for 13 Yealink phones was approximately £1,600 total.

What were the results?

Telephony costs fell from approximately £1,840/month (Mitel maintenance, ISDN lines, call charges) to approximately £1,140/month (Teams Phone licences, Operator Connect, compliance recording) — a saving of approximately £700, a 38% reduction. The approximately £1,600 hardware investment was recovered within 3 months of savings.

Beyond cost, the mobile app removed the missed-call problem the Mitel system had caused, since it offered no mobile capability. After 60 days, hybrid workers reported that having calls, meetings and messages in one app made their day noticeably easier. The IT manager found day-to-day changes — extensions, call routing, voicemail — simpler in the Microsoft 365 environment than the legacy PBX. The hybrid-first approach (laptop plus mobile app for most users) saved approximately £1,800 in hardware versus deploying a phone for everyone.

A practical note: the compliance recording partner licence added approximately £3.50/user/month for the 6 covered users. Factor that in early — it is not an afterthought. The same applies to porting: the 8-day window needed careful coordination with the legacy provider, made more pressing by the wider PSTN switch-off that Ofcom and Openreach are driving across the UK.

AMVIA managed this engagement end to end — licensing assessment through go-live support and staff training — and applied the same Microsoft Teams security controls that protect any Teams Phone deployment.