Why not use a voice modem as FXO card?

Hi

%09I browsed through the archives, but didn't find a very good
explanation as to why we can't use a good voice modem as FXO card for
small setups (read: cheap).

Is it because their CODECs suck, or they don't support enough of them?

Thanks.
Telecom, Cabling and VoIP continues...