->config->SIP(General)->change the local port as you wish, then restart the server(you'd better restart the PC), then you'll find the port changed and work normally. We have tried for many times.
Don't concentrate on the sip server/pbx as your problem. Focus your attention on the ua and make sure it is set up properly for the different port. Have you tried a softphone? Do you know what settings to change on your ua to make it work with port 5061?