Brian,
Have you looked at this document:
http://www.net.com/Product%20Literature/MicrosoftUC/VX_UC_QSG_4_4_4.pdf. It¿s Microsoft but they do have some mappings of SIP to Q850 codes which are based on the RFC¿s. The 127 disconnect code seems to be the code used when the system can¿t determine the precise cause.