One way Audio on SCCP - Outbound Calls

Hello friends, good morning!
I have a server with 39 SCCP extensions, all Cisco 7945 and 7965 phones.
A strange problem has been detected for a few days - when SCCP Extensions are making outbound calls to mobile numbers, the caller cannot hear who they are calling.
But the mobile number receiving the call can hear who is calling.

This is only happening with mobile numbers, and only with SCCP Extensions (PJSIP Extensions work normally for all outbound calls).

Strangely, we noticed that our voip provider has (I don’t know if this is a feature or a bug) a peculiarity - if a call is not answered and the destination does not have a voicemail (like my cell phone), instead of ending the call the provider makes a new call to the number.

For example - if I call through this trunk, and my cell phone doesn’t answer the first time, it will ring for a second, third, fourth time… until the cell phone is answered or the caller ends the call.

And we noticed that when this happens, the audio works normally (two way audio, working perfectly).

I’m sure it’s something with my provider, maybe something related to CODECs or reinvites (i think… since on the second contact attempt, the audio through the SCCP goes through normally?) but they already checked and said there’s nothing wrong.

The server has the latest version of FreePBX and the updated modules, my trunk is PJSIP, the router/firewall has ports 5060, 10000-20000 directed to the server (only coming from the provider’s IP), and all extensions SCCP has “nat=yes” in its configuration.

Do you have any idea what could be causing this problem or any direction I can give the VoIP provider to check?
Or could it be something internal to the network?

The problem was in CODEC configuration :laughing:

Comparing the channels in the Asterisk console I noticed that the outbound calls were like this:

NativeFormats: (g729)
WriteFormat: ulaw
ReadFormat: g729
WriteTranscode: Yes (ulaw@8000)->(slin@8000)->(g729@8000)
ReadTranscode: No

and the inbound calls (which worked perfectly) like this:

NativeFormats: (ulaw)
WriteFormat: ulaw
ReadFormat: ulaw
WriteTranscode: No
ReadTranscode: No

I found it strange to use the g729 codec on outbound calls and changed the settings to only allow alaw and ulaw.
After that, it was no longer necessary to transcode and the audio started to work perfectly on both ways!

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