PBX rejects the call with the Reason: Q.850;cause=88;text="INCOMPATIBLE_DESTINATION"

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 a TLS/SRTP connection, try to switch to the UDP protocol in the Ringotel Connection settings and try the calls again.