This problem is still un-resolved and if anybody have any suggestions here, I would appreciate it. Can there be some limit somewhere in the programming as to how many dialed digits the system takes from the 3rd party SIP phone? That's the only thing I can find different between calls working and calls not working: the working ones have max 6 digits (the longest extension), while the non-working call is composed of 8 digits (access code + 7-digit local number) or more.
But then again, I can be off with this idea...