As I mentioned before, Linksys is now selling cheap SIP analog telephone adaptors using a design licensed from Sipura. There was a long-standing question about the Linksys PAP2: does it only work with Vonage, or will it work with any SIP provider?
Initial reports were spotty–some people claimed to have their PAP2 working with Asterisk, while others found their PAP2 automatically connecting to Vonage. According to a few posts on the asterisk-users mailing list, the answer is tied to the model number: if you get the PAP2, you might be stuck with Vonage. If you buy the PAP2-NA, then you’ll end up with a provider-agnostic device. They’re both around $50, although the -NA is currently harder to find.