[sip] SIP text messages (SIMPLE) are not received/sent.

Tested using the latest F-Droid build (20191208-01):

1. Register a SIP account, e.g. from the voip.ms provider, which implements a SMS <-> SIP SIMPLE gateway.
2. Send a text message using another SIP account, or alternatively if using voip.ms with SMS support, a SMS from a mobile phone.

Actual: No SIP message gets received nor sent. Expected: SIP messages should be received/sent.

FWIW, I believe the protocol used for the text messages over SIP is SIMPLE (see: https://en.wikipedia.org/wiki/SIMPLE_%28instant_messaging_protocol%29).