FAQ/How to fix my Snom phone's audio problems with Fritzbox?

Answer

In older Fritz!Box firmware versions you might experience bad audio quality with snom phones which are being caused by interoperability problems due to packet size.

Updating the Fritz!Box

You should make sure, your Fritz!Box is running the latest or at least a current version. If you experience audio issues when using snom phones in combination with Fritz!Box, please perform a firmware update as explained in the AVM FAQ.

Change phone settings

Users have reported that modifying the following phone settings has fixed audio issues:

• Symmetrical RTP: ON
• RTCP Support: OFF

Configuring packet size manually

In case a firmware update of the Fritz!Box is not fixing the audio problem, you might try a different approach to this problem.
Please note, that this procedure can have undesired side effects and should only be applied by experienced users!

Packet Size on the Snom phone

Make sure the Packet Size is set to 20ms on your Snom phone

Packet Size on the Fritz!Box

The following interoperability problem in the packet size settings in Fritz!Box has been found to affect the audio quality of the calls: the Fritz!Box does not add any 'ptime' parameter in the SDP while negociating RTP, but it sends RTP with a packet size of 30ms. In order to fix this, you must activate telnet on the Fritz!Box and change this setting manually in the configuration file.

• To activate telnet, we found the following article to be helpful: http://www.wehavemorefun.de/fritzbox/Starten_von_telnetd
• Once you have activated telnet service on your Fritz!Box, edit the file /var/flash/voip.cfg by running command:

nvi /var/flash/voip.cfg

• Type / to search for the text "tx_packetsize_in_ms"; type i to enter edit mode; change the setting to 20; then press ESC to return to command mode
• Repeat previous step to change the "tx_packetsize_in_ms" to 20 everywhere it appears
• Type :w to save the file, and :q to exit the editor.
• Type reboot to restarts the Fritz!Box.