Sounds like a trouble with the Asterisk.
I tried that before, sending calls from a Quintum to a IVR system and worked.
The other posibility existed on the old A400 and A800, but I am not sure about the new ones.
It was called “ip extension”
Instead of sending the calls to a “number”, the calls get sent to a I.P. addrees and a port number.
I do not know if they did away with this function on the new ones or still there under a diferent name.
Maybe someone else have that info and could help.
Best luck!!!
Wilson Boyrie.