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

Background: A Growing Firm Outgrowing Its Phone System

A 35-person professional services firm based in the East Midlands had been running a Mitel on-premise PBX for seven years. The hardware was approaching end of support, maintenance costs had increased year-on-year, and the system offered no mobile working capability — a significant gap for a firm whose staff regularly worked from client sites and home offices.

The firm was already a Microsoft 365 Business Premium subscriber. Teams was in daily use for internal meetings and project collaboration. The question for the IT team was whether Teams Phone could replace the Mitel PBX entirely, or whether a separate hosted VoIP platform was needed.

Assessment: Teams Phone vs Hosted VoIP

AMVIA conducted a requirements assessment covering call volumes, feature requirements, number retention, and user working patterns. Key findings:

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

The assessment concluded that Teams Phone with Operator Connect was the right fit. The existing Microsoft 365 commitment meant Teams Phone licences were incremental to existing spend rather than a completely new platform cost. The heavy Teams usage among staff meant minimal additional training would be needed.

Deployment Approach

Phase 1: Licensing and Carrier Selection

Teams Phone add-on licences were procured for all 35 users. AMVIA evaluated Operator Connect carriers available in the UK market, selecting a carrier offering competitive per-minute rates with a strong UK network SLA. Operator Connect was chosen over Microsoft Calling Plans based on call volume analysis — at 4,200 minutes per month, the carrier's per-minute Operator Connect pricing was more cost-effective than Microsoft's bundled Calling Plan tiers.

Phase 2: Number Porting

All 9 existing numbers (main plus 8 DDI) were ported from the legacy Mitel/PSTN provider to the Operator Connect carrier. The porting process ran over 8 working days, with all numbers porting simultaneously on a pre-agreed cut-over date. A brief parallel running period was arranged to confirm all routing was working before the PBX was decommissioned.

Phase 3: Auto-Attendant and Call Queue Configuration

The main inbound number was configured with a Teams auto-attendant: business hours greeting and routing to a receptionist call queue, with overflow to voicemail outside business hours. A separate after-hours attendant played a message with key contact information. Six staff members received direct dial numbers mapped to their Teams Phone extensions. Two call queues were configured for the two client-facing teams with highest inbound call volumes.

Phase 4: Compliance Call Recording

The firm's compliance requirements covered two specific teams (6 users in total). A Microsoft-certified compliance recording partner was deployed for these users. All calls on the 6 relevant extensions are now automatically recorded, stored with tamper-evident controls, and accessible via the compliance recording portal. The remaining 29 users operate without call recording.

Phase 5: Device Transition

The 35 users fell into three categories:

• Office-primary users (12): Yealink Teams-mode IP phones deployed at desks
• Hybrid workers (18): Teams desktop client on laptop + Teams mobile app on business mobile. No desk phone deployed.
• Receptionist (1): Yealink T54W Teams-mode phone with physical presence display for monitoring team extension status

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

Outcomes

Cost Reduction

Previous monthly telephony cost (Mitel maintenance, ISDN lines, call charges): approximately £1,840/month. New monthly cost (Teams Phone licences, Operator Connect, compliance recording): approximately £1,140/month. Monthly saving: approximately £700 (38% reduction). Hardware investment (Yealink phones): approximately £1,600 — recovered within 3 months of savings.

User Experience

Staff feedback after 60 days was strongly positive. The mobile app eliminated the missed call problem that had been a persistent issue with the Mitel system (which had no mobile capability). Hybrid workers reported that having calls, meetings, and messages in one application made managing their day significantly easier.

IT Administration

Teams Admin Centre replaced the Mitel PBX admin portal. The IT manager reported that day-to-day changes — adding extensions, adjusting call routing, managing voicemail — were noticeably simpler in the Teams Admin Centre than in the legacy PBX interface.

Lessons Learned

Three points worth highlighting for businesses considering a similar deployment:

1. Compliance recording adds meaningful cost: The compliance recording partner licence added approximately £3.50/user/month for the 6 covered users. This is worth factoring into the cost comparison early — it is not an afterthought.
2. Number porting planning is critical: The 8-day porting window required careful coordination with the legacy provider. Start the porting process earlier than you think necessary to avoid delays to the go-live date.
3. Device decisions are user-specific: The hybrid-first approach (laptop + mobile app, no desk phone for most users) saved approximately £1,800 in hardware cost compared to deploying phones for everyone. For knowledge workers on laptops, a headset and the Teams client is a better fit than a desk phone they rarely use.

AMVIA managed the end-to-end deployment for this client, from licensing assessment through to go-live support and staff training. If your business is evaluating Microsoft Teams Calling, AMVIA provides the same fully managed service.