PSTN vs PBX: What Is the Difference and Which One Do You Need?

PSTN vs PBX: What Is the Difference and Which Do You Need?

PSTN (Public Switched Telephone Network) is the traditional landline phone infrastructure owned by carriers. PBX (Private Branch Exchange) is a private switching system your organization controls. Most enterprises in 2026 are replacing both with Microsoft Teams Phone, which delivers cloud-based PSTN calling without on-premises hardware. EPC Group has implemented enterprise telephony for 29 years.

PSTN vs PBX: comparison at a glance

Factor	PSTN	PBX	Microsoft Teams Phone
Infrastructure	Carrier-owned copper/fiber	On-premises hardware	Cloud-based, no hardware
Call routing	Carrier-managed	IT-managed switchboard	Policy-based in Teams Admin
Monthly cost	Per-line fees ($20–$60/line)	Hardware + SIP trunks	$8–$15/user/month
Scalability	Slow (new lines take weeks)	Limited by hardware capacity	Instant, software-only
Failover	Carrier-dependent	Requires backup hardware	Mobile app over cellular
Microsoft 365 integration	None	Limited (via SBC)	Native — calls in Teams
Best for	Legacy environments	Transition phase	Modern enterprise

Key facts

• PSTN lines cost $20–$60/line/month. Teams Phone Standard costs $8/user/month.
• Teams Phone with Calling Plan Domestic: $15/user/month, includes 3,000 outbound minutes/user.
• Operator Connect lets you keep your existing carrier (BT, Verizon, Lumen) inside Teams.
• SBC (Session Border Controller) bridges legacy PBX to Teams Direct Routing.
• Full PSTN migration to Teams Phone takes 16–31 weeks depending on user count.
• EPC Group holds core Microsoft Solutions Partner designations.

What is PSTN?

PSTN stands for Public Switched Telephone Network. It is the traditional carrier-owned telephone infrastructure — copper and fiber lines managed by providers like AT&T, Verizon, and BT.

PSTN connections are billed per line. Adding capacity requires a carrier order. There is no integration with Microsoft 365 or Teams without a bridge device.

What is PBX?

PBX stands for Private Branch Exchange. It is a private telephone switching system your IT team operates. It routes internal calls without using carrier lines and connects external calls through SIP trunks or PSTN lines.

Traditional PBX requires on-premises hardware. IP-PBX uses VoIP over your network. Both require IT management and hardware refresh cycles every 5–10 years.

Microsoft Teams Phone: the modern replacement

Teams Phone replaces both PSTN lines and on-premises PBX with a cloud calling system. There is no hardware to buy or maintain. Calls live inside the Teams app your employees already use.

• Teams Phone Standard — $8/user/month. Outbound calling via Microsoft Calling Plans.
• Teams Phone with Calling Plan Domestic — $15/user/month, 3,000 outbound minutes/user.
• Operator Connect — keep your existing carrier. Integrate natively with Teams.
• Direct Routing — connect any SIP carrier to Teams via a certified SBC.

Failover and reliability

Enterprises worry about calling continuity when internet goes down. These mitigation options apply to Teams Phone deployments.

• Dual ISP with automatic failover — route calls over a backup internet connection.
• SD-WAN — intelligent path selection for voice quality.
• Survivable Branch Appliances (SBAs) — local PSTN connectivity during Teams outages.
• Mobile Teams app — calls over cellular data as a fallback.
• Call forwarding rules — redirect to mobile phones automatically during outages.

PSTN migration to Teams Phone: phases

A typical enterprise migration from PSTN/PBX to Teams Phone follows six phases.

1. Assessment & planning — 3–4 weeks. Audit existing lines and auto attendants.
2. SBC deployment & SIP trunk configuration — 2–4 weeks.
3. Auto attendant & call queue design — 2–3 weeks.
4. Pilot group migration — 3–4 weeks. Validate call quality and routing.
5. Phased production migration — 4–12 weeks based on user count.
6. Legacy PBX decommission — 2–4 weeks after final migration.

Why EPC Group for telephony migration

• Microsoft Solutions Partner — core designations, including Modern Work.
• Oldest continuous Microsoft Gold Partner in North America (2003–2022).
• Expertise in Teams Phone, Direct Routing, Operator Connect, and SBC configuration.
• HIPAA, FedRAMP, and SOC 2 compliant implementations for regulated industries.

Frequently asked questions

What is the difference between PSTN and PBX?

PSTN is carrier-owned public telephone infrastructure. PBX is a private switching system your organization controls on-premises. PSTN provides the phone lines. PBX routes calls between those lines and your internal extensions.

Is Teams Phone a replacement for both PSTN and PBX?

Yes. Microsoft Teams Phone replaces traditional PSTN lines and on-premises PBX. It delivers calling through the cloud. No hardware is required. It integrates natively with Microsoft 365 and your existing Teams environment.

How much does Teams Phone cost vs PBX?

Teams Phone Standard costs $8/user/month. The Domestic Calling Plan adds $15/user/month. Legacy PBX maintenance, SIP trunks, and per-line PSTN fees typically run $30–$80/user/month. Most enterprises see 40–60% cost savings after migration.

How long does a PSTN-to-Teams migration take?

A full enterprise migration takes 16–31 weeks: 3–4 weeks of planning, 2–4 weeks of SBC setup, 3–4 weeks of pilot, 4–12 weeks of phased production migration, and 2–4 weeks of legacy decommission.

Can I keep my existing phone carrier with Teams Phone?

Yes. Operator Connect lets you keep carriers like AT&T, Verizon, BT, and Lumen while integrating their service natively into Teams. Direct Routing works with any SIP-certified carrier using a Session Border Controller.

What is an SBC in telephony?

A Session Border Controller (SBC) is a device that bridges your existing SIP phone infrastructure to Microsoft Teams. It handles call signaling, security, and media transcoding. EPC Group configures certified SBCs from AudioCodes, Ribbon, and Oracle.

Schedule a Teams Phone consultation

Talk to an EPC Group telephony architect about migrating from PSTN or PBX. Call (888) 381-9725 or request a 30-minute discovery call.