It is most often the problem with the configuration of the audio codecs. Therefore, please check the supported codecs on the PBX and configure your Ringotel Connection accordingly.
Alternatively, in In a TLS/SRTP connection, disable SRTP protocol or ensure that SRTP is properly configured on the PBX.
Alternatively, try to switch to the UDP protocol in the Ringotel Connection settings and try the calls again.
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