FXS - Foreign eXchange Subscriber interface (the plug on the wall) delivers POTS service from
the local phone company’s Central Office (CO) and must be connected to subscriber equipment
(telephones, modems, and fax machines). In other words an FXS interface points to the
subscriber

FXO - Foreign eXchange Office interface (the plug on the phone) receives POTS service,
typically from a Central Office of the Public Switched Telephone Network (PSTN). In other words
an FXO interface points to the Telco office

Cheers!

For VOIP? Hop on hop off arrangement? Sipura 3000 fits the bill here. It is complicated to setup but is worth it. Grandstream makes a few models with both fxs and fxo in the same box. Give me a clue as to what you want to do and I will tell you what I know.

Feb 28 8:12 PM - Is the VOIP end coming to you from an ATA that the provider has sent you? Does that provider allow you to connect to their system without their box? Just a few more details and I am sure I can steer you right. Will callerid make a difference to you? Are you setting up an asterisk pbx? A good description of how you are going to hook up would help. For instance, my system is both a land line and a VOIP provider ATA. Asterisk is used to provide dialtone to 6 extensions in the house and to tie both the landline and the VOIP line in. I can call in the landline and get dialtone to call out the voip line. Ring 1 or all of the extensions in the house. Listen to a house monitor (microphones). It gives me the additional ability to select what hours my phone will ring and by whom. My setup cost me about $260 /not including the phones themselves or the computer to run asterisk. Asterisk is embeddable in a Linksys router or NAS so you could add that for another $80. The Sipura 3000 can be had for about $90 and come with 1 fxo and 1 fxs port.