I have used spa502's behind cisco 2600's Mostly we use Adtran 90x T1 routers though. I just had basic nat turned on with dhcp. We have our Acme as our Outbound Proxy and they worked fine. The problem we run into most often is that the customer has sipALG turned on in their cheap router.
To get around this we set static ips on our ip phones 10.1.x.x and have that space routed in our whole network. We then plug one cable into the wan side of their router and one into the lan side. Their data goes out the wan connection but the phones go out the lan connection. This setup even works on customers who have Sonic Walls which, in the past we would not take them as customers.
