There are such devices. They are called a PBX (Phone system) and FXO VOIP Gateways. The commerical units sell for several thousand bucks a pop. So that is not a good solution for home.
However, there are always alternatives.
Microsoft and Linksys have aninexspensive SIP PBX, and there are many linux based ones as well. With the Linux ones you will need an old PC and a PCI FXO analog card($$). Switchvox has a free PBX (LINUX) and that is what I am using at home. I got my card from NetxUSa.com.
Unfortunately It is not just a matter of converting analog to SIP. SIP is just a protocol and requres contact and authentication to allow it to function. The conversion and control happens in the PBX.
Linksys is most likely your cheapest bet. It is not too fancy and will do the basics. It is also fairily easy to configure. I cannot be certain it will work with your phone though.
SPA9000: $ 399.99
<a href=”http://www.linksys.com/servlet/Satellite?childpagename=US%2FLayout&packedargs=c%3DL_News_C2%26cid%3D1136499819516&pagename=Linksys%2FCommon%2FVisitorWrapper”> An article on that is here.
Hope this helps,