While evaluating whether to use my Sipgate VoIP number as my fax receiving number 24/7 (with area code instead of the dus.net costly 01805) I stumbled upon a notice platform. Sipgate prints the whole string given as an argument to the url
http://www.sipgate.de/user/index.php?message= as “welcome message”. Look for yourself. If you’re looking for escape sequences and don’t mind to ignore the German bits stop by at user-archive.de.
Concerning the fax question I couldn’t figure out whether sipgate would pass fax data as fax data via VoIP so my fritz!box could actually handle the incoming fax correctly. Haven’t had time to just test it. But if so I’ll report it.