making a voip telephone call through fxos that have the signaling protocol referring to fig2 an fxo 20 fxo a interfaces with a voip user at company a similarly another fxo 20 fxo b interfaces with a voip user at company b the fxos 20 have the signaling protocol discussed above for purposes of discussion the voip user at company a user a places a call to the voip user at company b user b however user a does not know that user b has voip capability therefore user a calls user b using a pstn number and a pstn connection because the telephone call is made to the pstn number fxo a converts a digital audio signal to an analog audio signal for input to the pstn fxo b answers the telephone call because the telephone call comes in on the pstn because fxo b has the signaling protocol fxo b sends a message to fxo a to determine if user a has voip capability the message is referred to as a u201c voip capability check u201d fxo a receives the message and sends an answer back to fxo b the answer includes information that user a has voip capability fxo b receives the answer and sends the internet telephone address for user b to fxo a fxo a.