[Serusers] Using SER to bypass port-5060 filter

My ISP, which holds both the broadband and landline telephony monopolies in this area, blocks port 5060. Also, I am behind NAT (at this location; my router has a real address). I am hoping it would be possible to set up a proxy at a 3rd location to make it possible to use the Broadvoice service. I am able to use FWD for incoming and outgoing without trouble, so I don't think there are any other obstacles. I tried setting up my 3rd-location server to simply reflect packets that hit it on UDP port 7000, so they went to Broadvoice's server on port 5060. Unsurprisingly, I suppose, that didn't work. Tcpdump showed that nothing ever came back along that route; I am not sure whether Broadvoice's server was sending replies anywhere, but they sure weren't making it to my intermediate reflecting machine. So I guess there needs to be some rewriting going on. It seems like SER may be the tool for this, but after looking through many example configurations, I was unable to find something as simple as what I'm after. Does anyone have any ideas about a good place for me to start? Is SER in fact an appropriate tool for this task? Thanks. miguel