There are ports for sending packets to your OBi and ports for sending packets from your OBi to your voip provider. You can easily change the port for sending packets to you, you just change X_UserAgentPort under VoiceServices-->SPx Service"-->X_UserAgentPort. Change it to most any number you wish. In addition, your router may be changing the port without your knowledge in its NAT mapping scheme.
Changing the port number for packets you send to your VoIP provider is a different story. Your VoIP Provider must indicate that they are listening for the new port number. The default port number for sending to your voip provider is 5060 and is commonly referred to as the standard port. If you wish to send to a different port you add the port number to the proxy address. For example if the proxy address was
sip.anveo.com and you wished to send to their port 5010 you would send to
sip.anveo.com:5010.
VoIP providers that I am aware of that allow non standard incoming port numbers include Anveo (5010), VBuzzer (80). Hopefully there are more, I would suggest you ask your voip provider if there is an alternative to port 5060 for sending to them.
PBXes, a managed PBX service, that is an alternative for you to use, allows a wide range of incoming port numbers they call "alternative ports". You can register your ata to an account with them using a non-standard port and then setup your voip provider as a trunk with them using the standard port 5060. PBXes service is without charge for usage less than 2000 minutes per month.
Edit: Anveo uses port 5010 for their proxy address