The first thing to do is to have your technical people take SIP traces of those calls and see what is happening in the protocol trace.
Install Wireshark on the same pc you use to make the phone calls to analyze what is happening
As a telecom operator, you should have some technical people capable of analyzing sip traces on your payroll.
I worked in a VoIP engineering function at a large telecom operator in Belgium, and I can tell you that normalizing the number format of
called and calling number formats in your VoIP system is very important to make it work even if your clients send numbers in different numbering formats (E164, national with and without leading zero, ....)
This will save you tons of support calls to your helpdesk.