Cisco ASA 5520 Unable to reserve port 5060 UDP

Port forwarding and filtering are done with Access Control Lists (ACLs). Your NAT configuration simply provides a 'public' IP address for an internal device so that it can get out to the public Internet.

I'm making assumptions, here. I'm assuming UDP 5060 is inbound to your network. If so, your ACL should look something like this:
access-list outside-in extended permit udp any 192.168.x.x 255.255.255.0 eq 5060

This assumes that the name of the ACL is "outside-in" and it's applied to the outside interface. It assumes that the source traffic can come from any source, while the destination address is a standard non-routable subnet (192.168.x.x) with a 24-bit subnet mask.

Hope this helps...

Mmm that error is usually seen when you try and forward a range of ports and you have particular versions of the OS?

At the end I solved by myself