Wireshark can decode SSL/TLS sessions when the following conditions are fulfilled:
the private key of the TLS server is known (maybe both keys are needed if mutual TLS (=client certificate) is used?).
the TLS connections does not use a Diffie-Hellman cipher
Wireshark captures the TLS session from the beginning (handshake)
Configure Wireshark to decode TLS:
Edit → Preferences → Protocols → SSL → RSA Keys List: e.g.: ip.address.of.server,5061,sip,c:\key.pem
If you have problems decoding the TLS session you should enable debugging in Wireshark: Edit→Preferences→Protocols→SSL→SSL Debug File