About SIP port

in GUI Main >> Shared Objects >> Port Lists, you can create a defined SIP port range 10000-20000,

then create a sip pool monitor use gateway_icmp

finally, create a VS its "Service Port" use port list SIP  and binding a sip pool

if above steps can not work, you can create a vs service port use *All ports and bind this iRules to limit udp sip port range:

when CLIENT_ACCEPTED {
    if { [UDP::local_port] >= 10000 && [UDP::local_port] <= 20000 } {
        node 192.168.10.1
    } else {
        drop
    }
}