You could use wireshark to debug RTP traffic. Depending on what SBC you are using, here is the instructions for audiocodes.

https://www.audiocodes.com/media/13565/audiocodes-quick-reference-guide-how-to-debug-voice-with-wire...

 

Does the call disconnect when you don't hear any audio? I think there is some error made in the setup of your SBC.

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