The presence module must remember the URI in the To: header of a SUBSCRIBE request and copy it to the From: header of an outgoing NOTIFY request. This works for sip URIs but fails for tel URIs.
Actually it parses the URI as user@domain and stores the user & domain seperately in the database so it looks like it was not designed to work with tel: URIs.
The attached patch implements a workaround: a tel: URI will be saved with the telephone number in the username field and a blank domain; when this information needs to be converted back to a URI, a blank domain signals that a tel: URI should be generated. Actually this is OK since a sip: URI can never contain a blank domain.
With this patch, it is possible to subscribe to a presentity for presence using a tel: URI.