IP Phone Can Receive Calls but Failed to Make Calls - MTU Limit

Applicability

• Model: S-Series, P-Series (Including cloud PBX)
• Firmware version: Any 

Symptom

• IP Phone can receive calls but failed to make outbound calls.

Cause

Most of the cases are due to MTU limit of the router on the site where the IP phone located.

If you capture the Pcap log on the PBX, and review the VoIP Call flow. You will see IP phone doesn't send the re-INVITE after PBX sends 401 code. Because the local network MTU limits the re-INVITE packet.

A normal call flow is like this:

Solution

Choose either solution based on your need.

1. Reduce the codec for the IP phone.

If your phone was provisioned by PBX, go to the Auto-Provisioning menu to reduce the codec. Only leave the alaw and ulaw.

Then re-provision the phone and make call test to if it works.

2. Change the extension registration type to TCP.

The MTU limit is for UDP protocol, if we set registration to TCP it will have such issue.

Refer to this article to change the registration type: How to Register Extension by TCP on P-Series