I do not know if it will help you entirely, but I had similar case here in Moscow where we have connected PABX Hicom300 (SIEMENS) to PSTN Exhcange System-X (GPT technologies).
At that time we were told that System-X will provide ETSI protocol, so that we had programmed our PABX with the same one. Later I found that System-X was programed on Q931 basis, which is a older platform for ISDN functionality.
It works fine, but because in Russia Technical Conditions for the time being do not require ISDN features, that part was never ever tested. Never the less one problem in the the main group of signalling messages has been noticed, far end B busy suscriber case: System-X generates Alerting message and through connect speach path instead of sending Disconect message.
There were some other problems as well related to acoustic signals and speach path but I am not sure if that was related to this matter. However software that is used on your site can differ from described one. So I wish to you much luck and better to use real ETSI.