A simple NAT configuration would work just fine.
https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-concepts-11-4-0/17.htmlconceptid
But you could do better with an actual virtual server configuration. As a full proxy, the F5 will always proxy layer 4 (TCP and UDP), but beyond that layers 5 and up are controlled by profiles that either manage the traffic or simply pass them through. A simple virtual server listening on port 5060 or 5061, with nothing more than a pool assigned also pointing to port 5060 or 5061 backend services will not terminate or touch the SIP traffic passing through it.