Actually to connect PSTN lines (regular telephone lines coming from your telecom provider) to Asterisk you only need FXO cards. Or ATA’s (analog telephone adapters) – specially if your Asterisk box doesn’t have PCI or PCI-e slots. The FXO cards can be PCI or PCI-express (or other flavours for more complex setups) with 1-8 (or even more) connectors per card for the same number of telephone lines to be connected to Asterisk. Digium, Sangoma, OpenVox and others make them. You only need FXS cards if you have analogue phones which you want to re-use. Otherwise there are many manufacturers of SIP hardware phones – looking perfectly similar to regular business desk phones – so there isn’t much point IMO to buy FSX cards unless you already have the analogue phones and need to re-use them.