Another vote for Voip.ms listed above. I do exactly what the OP wanted - I have a couple of Linksys $30 ATA devices and the one phone number rings multiple devices at once and they all share the same voice mail, caller-id, etc, etc. Exactly what you want.

I've been using these guys for a little over two years, very happy, I've heard Callcentric is another decent one.

My monthly bills for all this is well under $5 a month total cost for all my phone usage. My monthly phone bills went down from about $30 a month to under $5, usually more like $3 a month, with a ton of customization available.

I was a little intimidated setting it up at first, but it's actually pretty easy. I also put a SIP client on my iPhone so when someone calls the home number it rings two locations plus my iPhone, so three devices, one phone number, no call-forwarding or anything like that. When traveling internationally I can use the iPhone SIP client to call outbound and the caller-id shows up as my home number and if anyone needs to reach me they can, for no additional cost or international charges.