A Q.931 call has the 'transfer-cap' attribute set to 'udi' for data
calls. The ISUP module must pick this up and encode it in the
UseServiceInformation IE to preserve this vital information. Inversely,
a call arriving via ISUP must put its transfer-capability into the
'transfer-cap' attribute, so that when it's routed to Q.931 it is
preserved.
Without this transfer-capability transparency, no UDI/RDI data calls
or ISDN video calls can transition on the Q931/ISUP boundary
See also https://github.com/yatevoip/yate/issues/12
Harald Welte
3701992c74