In this example:

send 1149 bytes to udp/[<extension_ip>]:5060 at 14:47:46.972424:
  ------------------------------------------------------------------------
  INVITE sip:<extension>@<extension_ip> SIP/2.0
  Via: SIP/2.0/UDP 204.93.201.164;rport;branch=z9hG4bKDZKU0eQBtr3Be
  Max-Forwards: 64
  From: "UNKNOWN" <sip:<caller_id_number>@<FS_ip>>;tag=FFr3B9gSB0KZK
  To: <sip:<extension>@<extension_ip>:5060>
  ...

I need to change sip_invite_req_uri and sip_invite_to_uri so that the
switch downstream can route the call based on the DID they have with me.
I've tried setting this several different ways but no avail. I'd like it to
be global if possible. Is there a good spot to set these variables?