The sip port configured on your OBi for SP1 and SP2 is the port number for incoming packets being sent to you. You send outgoing packets to voip providers at their sip port which by default is port 5060. That doesn't have anything to do with the sip port that you configure on the OBi. If their sip port is not the default 5060 you must include their sip port in their proxy address. For example Anveo's proxy server is
sip.anveo.com:5010. Your voip provider will send packets to you at whatever sip port you configure.
Your sip port on your OBi adapter can be most any number, indeed many NAT routers change it without your knowledge.
You can configure Google Voice on SP1 and any sip provider on SP2. Looking at the VoipAlot web site it says you setup their proxy as
sip.voipalot.com. When the web site says SIP Port 5060 they are talking about their sip port. When it is 5060 you do not have to include it in the proxy address because 5060 is the default and the adapter adds it to the address.