How to modify SIP::URI

I want to modify ip addr (1.1.1.1) in sip uri :

 

INVITE sip:0596423330@1.1.1.1;user=phone;callmode=voice SIP/2.0

 

by the addr return by [LB::server addr]

 

function sip::uri just return the value

 

but how to set the value

 

is i need to parse and modify at udp::payload level ?