Enforce SIP Digest Authentication by IP PBX

If TLS cannot be configured (for whatever reason) and if you are using an on-premises IP PBX, it's crucial that your IP PBX implements SIP Digest Authentication for remote users. In addition, authentication should be applied to as many SIP methods as possible (i.e., not only on REGISTER messages, but also INVITEs, re-INVITEs, etc.).