In a Class 4 switch with all the trunks with CCS#7, the #7 signaling message IAM has a mandatory field called A-side category.
If we are replacing a Class 4 TDM switch with a VoIP solution with H323 signaling, the interworking CCS#7 – H323 looses the A-side category as this field is not defined in Q931 stack.
The outgoing call in the other side of the class 4 replacement must write in the outgoing IAM A-side category field, but as it was not transported, the outgoing gateway has to write down a default value, so, transparency is not possible, at least with pure H323 signaling.
Suppose that our Class 4 VoIP replacement receives a Q763 IAM message with A-side category “payphone”, this value is not mapped, so, the outgoing gateway has to build an outgoing IAM, but, as A-side category has not been informed, the outgoing Q763 IAM leaves the VoIP Class 4 with A-side category “Ordinary Subscriber”. If this call should be routed or billed according to A-side category, the solution is not usable.