Search under "USB to RJ11";you'll get a lot of hits. The problem is that most are as expensive as a USB phone handset such as
There is an inexpensive handset ($15/free shipping) which is identified as speakers/mike and mutes your PC speakers when in use.
Note the other hardware on the sipphone site. They include router/sip combos and sip adapters which do allow analog phone attachment. I'm not sure they will work with callserve.
Your best bet is to check the callserve site and grab one of the handsets that they sell on their site.