Hello Stan
The SP puts forward a number of requirements, such as national/international context in To: field, then some special requirements for CallerID privacy, etc. The problem is, we can't get a document that describes the technical details of the interface, and SP refuses to create such a document. All we've got is a number of emails and some information from phone conversations.
SIP and its extensions are fairly well standardized. Have a look at http://www.packetizer.com/ipmc/sip/standards.html for an overview of those RFCs. We all know the PBX manufacturers and their developers seldom fully comply to the standards so they should give you a good starting point on how it's supposed to be done. You will have to test each and every case with your SP unless he can garantee you he has implemented it fully standard compliant.
Is it a common situation for such a service? Am I too naive with my expectations to receive a fully documented service? If it were a no-name lousy cheap service provider, I wouldn't ask :)
We never had any issues when connecting SIP trunks to a provider as long as they were using RFC compliant SIP (IMHO the RFC compliance is a major decision point when choosing the SP). And I second you on the point that the SP should document its extensions to the protocol if they are not standard compliant extensions.
Regards
Jean-Pierre