If you had ISDN service you’d know it.
You almost certainly have common analog service from the PSTN (Public Switched Telephone Network).
You should look into the TDM400P with one FXO module. It will cost somewhere around $140-$150 US.
There are drawbacks and things to keep in mind when using analog service to an Asterisk PBX used in your home.
You’re home phones won’t start to ring immediately. It takes two ring cycles for callerID data to be delivered to the Asterisk PBX. Until it has all of that data, Asterisk doesn’t start to ring your in-home VOIP phones. Consequently, your callers will hear more rings than they are used to hearing before you answer your calls.
You don’t have any awareness of the state of the analog line from all phones. Right now, if you pick up an extension in your home, you’ll break into any call in progress. The Asterisk PBX can check the state of the line for you as you place calls, and react accordingly depending on the state of the outside line, and the type of call your placing, but you have to write all of that into the dial plan.
An example of point number two is emergency calls. (911) If you were to call 911, and the line were busy, what would you want the system to do? I’ve written routines that would check to see if a call to 911 was already in progress. If it was, it let it stay, and play a message that would tell the second caller that an emergency call was in progress. If not, it would interrupt the call, and dial 911 for you.
Build a system, and use it alongside (not replacing) your current telephone service for a bit. It’ll give you a chance to experiment, and figure out what you need it to do.