My VOIP provider has a SOAP API for sending SMS messages. (See <https://www.pennytel.com/api.pdf>). So sending the message is quite straightforward.

It's integrating with Contacts that I'm not sure how to do. Basically I want to add an "SMS via Pennytel" option for any mobile phone number assigned to a contact. Similar to the existing "SIP audio" call option which appears when you have a SIP account enabled. Can anyone tell me where this is handled?