In order to setup a SIP TLS connection in scenarios where the server requests a client certificate, the base must be configured with a pre-loaded TLS client certificate and key.
After clicking on Security, click on the "Choose Files" button under "Import SIP Client Certificate Key Pair"
The certificate must be provided as a DER encoded binary X.509 (.cer) file, and the key must be provided as an unencrypted PKCS#8 binary file.
If you need to convert a certificate in DER format you can use the OpenSSL command:
If you need to convert the private key into the unencrypted PKCS#8 format you can use the OpenSSL command: