Pipe sip into modem

hi there ppl :smile:
i was wondering, there are the softphones, which i can use to register at a sip provider and then use the computers mic and speakers to make calls. now asterisk of course, can also register at the sip provider but something like an ATA is required in order to be able to hook up a phone, now i was thinking wuold it somehow be possible to pipe the sound and mic stuff into a modem which is installed on the computer so i could just hook up a phone to my modem and use it like an ATA? that would be great as modems u can get for around 5$ or euros and ATAs r more expensive…
thx for thougths and tips on the this issue