Could you check if there are any existing connections and xlate enties for the port 5060 ?
sh connection | in 5060 and show xlate | in 5060
If there is any entry try clearing it using clear xlate/clear conn command and then try applying the static NAT.
! this should be tightened to allow traffic only from your telephone company, or
! people with bad intent will happily place international calls on your account
access-list outside_access_in extended permit udp any host 192.168.3.150 eq 5060
! nat 5060 to 5060
object network NEC_DSX
nat (inside,outside) static interface service udp 5060 5060
