The Sipura SPA3000 as mentioned will do what you want. Although it can be very confusing to configure, there are a few articles (Nerd Vittles, etc.) that have very good instructions on how to configure them to do what you want. Once you’ve had it - you can choose if you want to configure it to further customize it. (You can do one of the suggestions which is to have it act as the gateway to the phones and only pass to Asterisk if not answered, or you can go straight to Asterisk to manage the whole call (which is what I did and prefer).
One advantage of the Sipura 3000 approach over a Digium card is that it provides some good failover mechanisms. On power loss, it will mechanically connect the fxs-fxo port so if you have an extension on the fxs port and power is down, it will bridge the connection. (Just make sure that phone is not dependent on power like most cordless, or battery backed - or you still won’t have a phone:)) Also - if power is up and Asterisk isn’t responsive, you can have it fail-over to the FXS port as well.
For a home setup - this is a very good choice. (You can even use it to provide VOIP service if you decide that you don’t want to use Asterisk any longer).
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