custom headers support

[basav-raj]

I plan on using diagox as a proxy, however, my SIP server expects the following format in the contact header:

// Instead of: +sip.instance="<urn:uuid:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx>"  
// I need to have: +sip.instance="<xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx>"  

it is possible to achieve this?