I have some regular phones lying around the house and a pci card that has rj11 connectors in the back. I was wondering if I can connect the regular phone to the jack and make it act like it was a sip phone. I know there are all kinds of adapters out there that can do this but I was wondering if I can use the computer itself to make the regular phone into a sip phone.
What is that card ? IF it is modem answer is no . you need a DAHDI card .
It’s the card in the back of the tower that you plug telephone cables into it. I was hoping that you can turn this into the voip client. I am not talking about doing something on the server side (asterisk server need DAHDI cards to work but what about the clients) I have some computers in the house and some regular phones lying around, I was wondering if the computer can receive the call and relay all of the audio signals (and the ringer signal like a regular phone line would do) to the phone connected to the card in the computer. I don’t think a dahdi card is needed to do this since this isn’t happening on the server but the client that would otherwise be using a softphone but in this case all of the call signals are being passed to the rj11 card into the phone that is connected to it.
I am not using IAX2 protocol (except for maybe softphones being used outside the LAN). I am using SIP which I think doesn’t need any special drivers, codecs or firmware. it should just be a SIP driver for the card so that it can turn the phone into an IP phone that runs SIP protocol. Is this possible or do I have an overactive imagination 
Indeed, If you want to use clients you have some options: Use a SIP Client(softphone or Real IP Phone), use a FXS gateway it convert the signalling of your normal home phone to a SIP signalling(like PAP2T).
I was hoping to keep things in budget but now it seems simpler just to buy an IP phone that would cost me more than a hundred dollars. softphones won’t cut it since IP phones are more convenient for the less savy computer user. also availability isn’t an issue if ever someone decides to crash their computer leaving their softphone disabled until they can bring it back up again.
There are phones from $30USD and also ATAs in ebay 
I had some bad experiences with EBay in the past but where are you finding these phones for $30