Either use PCI-Cards in the same Machine where asterisk is installed or Use a LAN SWITCH and connect it with Asterisk Machine and then plug your IP Phones into the LAN SWITCH.
Well. You can use these cards with asterisk.
TDM10B – 1 FXS port
TDM40B – 4 FXS port
TDM01B – 1 FXO port
TDM04B – 4 FXO port
For more info you may contact me at sadaatahmad@hotmail.com