Generic UDP routing is kind of hard for multi packet protocols (like SIP or CoAP), you want to make sure packets belonging to same message are routed to same instance of your app. There are two ways to achieve this:
1) Protocol specific UDP routers (stateful) that understand the protocol and can route packets belonging to same message to same instance of your app.
2) Based on source IP route the packets to same instance of app (might not result in most optimal load balancing).

Anyways as James pointed out there is no support for UDP routing in CF. For your use case if it is an option for you to use SIP over tcp then tcp router, which is being developed for CF right now , can be an option for you.

Regards,
Atul