Asymmetric routing and VoIP

When running a VoIP system, network configuration is just as important as the phones and PBX software. One common issue customers encounter is asymmetric routing, which can break VoIP in subtle but frustrating ways. This article explains what asymmetric routing is, why it causes problems, and how to fix it.

What is asymmetric routing?

Asymmetric routing happens when your outbound and inbound traffic take different paths through your internet connections. For example:

Your VoIP phone may register through ISP #1,
But your router decides to send traffic back through ISP #2.

This mismatch leads to communication breakdowns:

Registration issues — the PBX tries to reach your phone on the wrong route.
One-way audio — caused by NAT mismatches.

Why dual-WAN and load balancing cause problems

VoIP requires a predictable, consistent path for traffic. If your router dynamically switches connections, SIP and RTP packets may arrive in unexpected ways, breaking call stability.

The fix: policy-based routing

To make VoIP reliable, you must force all VoIP traffic to use a single, consistent WAN path.

On your router, create a Policy-Based Routing (PBR) rule.
Ensure that traffic always uses one specific WAN interface (e.g. WAN1 – Rogers), matched by either:
- all traffic from the IP addresses of your SIP phones, or
- all traffic destined for your VoIP provider’s servers.

This ensures both SIP signaling and RTP audio packets take the same path, preventing NAT confusion and asymmetric routing. When done correctly, the router expects return traffic on the same interface it went out on, and everything works as it should.

See our best practices for setting up a router and/or firewall for the rest of the recommended configuration.

Additional consideration: Starlink and CGNAT

Some ISPs (like Starlink) use Carrier-Grade NAT (CGNAT).

With CGNAT, your connection does not have a true public IP.
Instead, you share an external IP with many other customers.
This creates double NAT (one at your router, one at the ISP), making VoIP traffic even harder to deliver back to your endpoint.

This is why VoIP often works poorly behind Starlink, but becomes stable when switching to an ISP like Rogers that provides a direct public IP.