Cisco IOS : inside local - router choosing lower port

I've got an IOS router which for some reason is choosing port 5061 for the inside local for an outbound SIP registration inside of a random higher level port.

I've tried a new IOS as I remember this being an issue on some IOS revisions but no luck

Usually this wouldn't be an issue but as the router is choosing port 5061 it's opening it up for dodgy IP's to come back in on.

I've put an ACL on the router to close it down but I also want to stop the router from choosing a common port for the nat translation.

Is there a way to remove the ports from being used during the nat tranlsation?

thanks