Supported protocolls are:
X	X75
u	ui	hdlc with UI frames
r	raw	hdlc without anything
C	BTX-X75	german CEPT X75 stuff
t	tel	ISDN and a/b telefon
I	ISDN telefon
a	a/b	a/b telefon

No tricks and thrills. I do not need T70 or other level 3 protocols.
I use IP.