Hi Klaus,
there is a patch on tracker to presence_dialoginfo for adding schema ( sip: ) to entity values in generated notify requests, see: - http://sip-router.org/tracker/index.php?do=details&task_id=324
Apparently it is correct, based on examples from RFC, but as you developed and tested, maybe you remember that was a different reason not to do so. The question is whether to make this a configuration option or just add schema always.
Cheers, Daniel