When the P-CSCF receives any 1xx or 2xx response to the above request (initial dialog request), the P-CSCF shall:
[...]
3) verify that the list of URIs received in the Record-Route header of the request corresponding to the same dialog is included, preserving the same order, as a subset of the Record-Route header list of this response. This verification is done on a per URI basis, not as a whole string. If the verification fails, then the P-CSCF shall either: [...]

TS 23.228, section 5.4.5 and TS 24.229, section 5.2.6.4 (version 7.8, 2007-06)