While setting up a SIP phone at home, I noticed something surprising - most SIP providers don't seem to offer any kind of encryption on the connection; both voice ...

Even many IT professionals haven't heard of the Session Initiation Protocol (SIP), so it's almost certain your users haven't. However, unless you're using an entirely proprietary Voice-over-IP (VoIP) ...

Using such an address will let a SIP connection set up a phone call but it won't be encrypted. To create an encrypted call, your device needs to add SIPS instead of SIP at the start of the address.