Sign with a provider that supports bring your own device options. This includes Viatalk, Callcentric, VOIP.MS, etc. With VOIP.MS you can actually program the two devices to work on the same DID. With the others, buy a $15 a year account wit Voxalot and program that to connect to your VOIP account. You can now connect multiple devices to Voxalot. They will use the same inbound and outbound connection. If you are worried about congestion issues, back this up with a PAYG outbound account for overflow. Look at VOIP.MS where you can set the outbound calls to spoof your caller-id.