How to Fix One‑Way Audio in PSTN to Microsoft Teams Calls: Root Causes & Step‑by‑Step Troubleshooting

If you’re experiencing one‑way audio in PSTN to Microsoft Teams calls—where either the caller or the Teams user can’t hear the other side—you’re not alone. This is a common real‑world issue in Teams Phone environments, especially with Direct Routing or carrier interconnects. The good news: in most cases you can resolve it by methodically validating media paths, firewall rules, Session Border Controller (SBC) settings, and call routing policies.

In this deep‑dive guide, you’ll learn how one‑way audio happens, how to diagnose it effectively, and how to fix the most common causes—covering Microsoft Teams media flow, SBC configuration, codecs, NAT, SIP/SDP, and network firewalls. Use this as a practical playbook to stabilize inbound PSTN audio for your Microsoft Teams users.

What Is One‑Way Audio in PSTN to Teams Calls?

One‑way audio means one party hears audio while the other hears silence. In the context of PSTN to Teams, this often looks like:

• The external PSTN caller hears the Teams user, but the Teams user can’t hear the caller (or vice versa).
• Audio works for internal Teams‑to‑Teams calls, but fails on some inbound PSTN calls.
• The issue appears intermittently—often tied to network conditions, specific SBCs, or certain carriers/routes.

Why One‑Way Audio Happens (The Short Version)

One‑way audio typically points to a media path problem rather than a signaling problem. Signaling (SIP/SDP) can succeed while Real‑Time Transport Protocol (RTP) flow is blocked or mis‑routed. Common culprits include:

• Firewall/NAT blocking UDP media (especially high ports and TURN/ICE traffic).
• Asymmetric routing or hairpinning at the edge or SBC.
• SBC misconfiguration (media anchoring, symmetric RTP, re‑INVITE/UPDATE handling, early media, RTCP multiplexing).
• Codec or DTMF mismatches between PSTN/carrier and Teams.
• SIP ALG enabled on firewalls, rewriting SDP incorrectly.
• TCP fallback for media due to blocked UDP impacting quality and directionality.

How Microsoft Teams Handles Media (Key Concepts)

Understanding how Teams media works helps pinpoint why one‑way audio in PSTN to Microsoft Teams calls occurs:

• Media Protocols: Teams prefers UDP for RTP media. When UDP is blocked, it may fall back to TCP/TLS, which can degrade reliability.
• ICE/STUN/TURN: Teams uses Interactive Connectivity Establishment to discover working media paths and may relay media via Microsoft Transport Relays (TURN) if direct paths fail.
• Codecs: Teams typically uses Opus for VoIP legs; PSTN legs commonly use G.711 (μ‑law/A‑law) on the carrier side. Transcoding often occurs at the SBC/carrier interconnect.
• Direct Routing: With Direct Routing, your SBC sits between the PSTN/carrier and Microsoft’s SIP proxies/media relays. SBC media anchoring and NAT handling are critical.

Fast Checklist: Fix One‑Way Audio Quickly

• Confirm UDP 3478–3481 (STUN/TURN) is open outbound to Microsoft 365, and high UDP media ports are allowed both ways per Microsoft guidance.
• Disable SIP ALG on perimeter firewalls/routers.
• Verify SBC supports and is configured for symmetric RTP, correct media anchoring, and NAT traversal.
• Ensure codecs are aligned (e.g., G.711 for PSTN side; SBC can transcode when needed).
• Check that early media and 183/180 handling is correct; avoid SDP inconsistencies on re‑INVITEs/UPDATEs.
• Test a problematic call and pull Teams Call Analytics + SBC RTP stats to confirm which direction is failing.

Step‑by‑Step Troubleshooting for One‑Way Audio

1) Reproduce the Problem and Capture Identifiers

• Identify a recent affected inbound PSTN call.
• Record the call ID, user, time, and PSTN number.
• Ask whether the Teams user could hear the caller or vice versa; note patterns (carrier, time of day, location).

2) Use Teams Admin Tools (Call Analytics & CQD)

In the Teams admin center, open the user’s call history. For the impacted call:

• Review media transport (UDP vs TCP), jitter, packet loss, and whether a relay was used.
• Check which leg lacks inbound RTP (Teams client RX) or outbound RTP (Teams client TX).
• If multiple users share symptoms, query Call Quality Dashboard (CQD) for trends by subnet, site, or SBC.

3) Validate Firewall & NAT Rules

Most one‑way audio cases stem from blocked or rewritten media. Verify:

• Outbound UDP 3478–3481 to Microsoft media relays is allowed from clients and SBCs.
• Allow required media port ranges (UDP high ports) as documented by Microsoft for Teams media.
• Ensure stateful return traffic is permitted and not timed out prematurely.
• Disable SIP ALG to prevent SDP rewriting that breaks RTP routing.

4) Inspect SBC Configuration (Direct Routing)

If you use Direct Routing, the SBC is pivotal in resolving one‑way audio in PSTN to Microsoft Teams calls. Examine:

• Media anchoring/NAT: The SBC should anchor media, advertise reachable public IPs in SDP, and honor symmetric RTP.
• SDP handling: Properly handle re‑INVITE/UPDATE, early media (183/SDP), and session refresh timers.
• Codecs: Ensure G.711 (μ/A‑law) is available toward PSTN; configure transcoding toward Teams if needed.
• RTCP multiplexing and ICE attributes should be passed or normalized per vendor best practices.
• Media bypass settings: test with bypass on and off to compare behavior.

5) Analyze SIP/SDP and RTP Statistics

On the SBC/carrier side, capture a call trace and confirm:

• Both legs (PSTN↔SBC and SBC↔Teams) negotiate matching audio streams in SDP.
• RTP flows in both directions; note if one side’s RX is zero packets while the other side’s TX increases.
• Check for asymmetric NAT or incorrect c= and m= lines (wrong IP/port advertised).
• Confirm no firewall pinhole timeout before the call is answered or re‑INVITE occurs.

6) Client, Device, and OS Checks

• Verify the Teams client has microphone permissions in the OS.
• Ensure the correct input/output devices are selected in Teams settings (esp. USB headsets or docking stations).
• Update Teams client and audio device firmware; test with a different endpoint to rule out device issues.

7) Carrier and Routing Factors

• If only some inbound calls show one‑way audio, identify the carrier route or specific DID range involved.
• Ask the carrier to validate codec negotiation, early media handling, and RTP anchoring in their network.
• Collect call examples with timestamps and call IDs to escalate efficiently.

Common Fixes That Resolve Most Cases

• Open/verify UDP 3478–3481 outbound and required media ranges; prefer UDP over TCP for media.
• Disable SIP ALG on all edge devices.
• Enable symmetric RTP and correct NAT mode on the SBC; ensure public IPs/FQDNs in SDP are reachable.
• Normalize SDP to avoid missing/incorrect ICE candidates and to pass RTCP mux cleanly.
• Standardize on G.711 toward PSTN and allow SBC transcoding toward Teams if required.
• Tune session timers (e.g., refresher=uas/uac) to prevent mid‑call media drops after re‑INVITE.
• Test with media bypass disabled; if it resolves audio, investigate NAT and routing asymmetry.

Edge Scenarios to Watch

• Early media only, no talkpath after answer: Often SDP re‑INVITE mismatch or firewall pinhole closing.
• Intermittent by site: Site firewall differences; CQD will show site/subnet correlations.
• Only external callers affected: Points to PSTN/SBC side, not Teams‑to‑Teams.
• Only certain DIDs affected: Carrier routing or SBC trunk group policy differences.

Preventing One‑Way Audio in the Future

• Document and enforce a Teams media firewall policy at all sites.
• Standardize SBC templates for Teams (codec set, SDP normalization, NAT, timers).
• Monitor health with Call Analytics and CQD dashboards; set alerts for anomalies.
• Engage carriers with clear test matrices (inbound, outbound, early media, re‑INVITE scenarios).

Internal Linking Suggestions (add links within your site)

• How to Open Microsoft Teams Firewall Ports and URLs
• Direct Routing: End‑to‑End SBC Configuration for Microsoft Teams
• Using Teams Call Analytics and CQD for Voice Quality Troubleshooting
• Media Bypass vs. No Bypass in Microsoft Teams: Pros and Cons
• Fixing SIP ALG Issues on Enterprise Firewalls

External Authority Link Suggestions (add links to these resources)

• Microsoft: Office 365 URLs and IP address ranges (Teams media and ports)
• Microsoft Learn: Plan Direct Routing and SBC requirements for Teams
• Microsoft: Use Call Analytics and Call Quality Dashboard
• Vendor Docs: Your SBC vendor’s Microsoft Teams interop and best practices guide

Conclusion