can someone give me an example in use, I have looked at the wiki and its pretty unclear, I am trying to remove a string in an invite -> ;phone-context=national

INVITE sip:403XXXXXXX;phone-context=national@10.185.16.170;user=phone SIP/2.0
  Via: SIP/2.0/UDP 10.185.16.169:5060;branch=z9hG4bKght6ecqg4eh7d5sluaj9cl2rr4
  From: "Caller"<sip:403YYYYYYY;phone-context=national@10.185.16.169;user=phone;isup-oli=00>;tag=SDj6q1a01-98964
  To: <sip:403XXXXXXX;phone-context=national@10.185.16.170;user=phone>
  Call-ID: SDj6q1a01-2afa0c6426482b0cfa2885f32878c86c-o0t3g30
  CSeq: 62218 INVITE
  Content-Type: application/sdp
  Contact: <sip:403YYYYYYY;phone-context=national@10.185.16.169:5060;transport=udp>
  User-Agent: Nortel SESM 14.0.6.0
  Max-Forwards: 19
  Supported: com.nortelnetworks.firewall,p-3rdpartycontrol,nosec,join,x-nortel-sipvc,com.nortelnetworks.im.encryption,r                                    eplaces,100rel,tdialog
  Allow: UPDATE,REFER
  x-nt-corr-id: 871f3514-29e7-1b21-a633-000e0cb7d3a0
  x-nt-location: -1
  Content-Length: 206
  Route: <sip:403XXXXXXX;phone-context=national@10.185.16.170:5060;user=phone;lr>


Thank You,
George