The Request-URI. This is the URI contained in the very top line of any request. In this example, the R-URI value is sip:+441869118000@biloxi.com;user=phone:
INVITE sip:+441869118000@biloxi.com;user=phone SIP/2.0 Via: SIP/2.0/TCP 1.2.3.4:5060;branch=xxxxx From: Bob <sip:bob@biloxi.com> To: Alice <sip:alice@atlanta.com>;tag=xxx CSeq: 1 INVITE Call-ID: 32f3-fk35-fkl45-fk45fg45
Note that the To header URI may be totally different from the R-URI in a random SIP message in a network. (See further down this FAQ)
Is it correct for a proxy to translate R-URI but not in the To: ?
That is correct. The To header is populated by the UAC, and represents the logical target of the request when it was generated, not it's current target on a hop-by-hop basis. That is what the R-URI (the request URI) is for.
What is the correct place to be looking for the peer/dialed digits; To: or the R-URI?
The R-URI is the correct place. A UAC can ostensibly put whatever value it likes into the To header - and that value is not allowed be modified by any proxy at all.
The To header field value should never, ever, ever be used for any form of routing in any type of SIP element. Ever.