Port forwarding on Cisco IOS devices can be a bit tricky but here is a easy way to do it.
192.168.59.10 is the server you want to port forward to. (You need to have this in twice for the NAT Pool)
ip nat pool POOL1 192.168.59.10 192.168.59.10 netmask 255.255.255.0 type rotary
ip nat inside destination list VOIP pool POOL1
!
ip access-list extended VOIP
permit tcp any any eq 5060
permit udp any any eq 5060
permit tcp any any range 10000 15000
permit udp any any range 10000 15000
