[Asterisk-Users] FXO vs FXS question

[Wolf Paul]

Hello all,

as a relative newbie I obediently ordered the recently recommended
"Newton's Telecom Dictionary" but it did not provide the answer to one
of my question. Perhaps one of you can do so.

What is the difference in functionality between an FXO and an FXS port?
I know that the line from telco goes into an FXO and standard POTS phone
goes into an FXS, but since both types of ports must be able to both
make and answer calls, what is the actual functional difference?

Or in other words, what does an FXO to FXS converter do (I have seen
some mention but no explanation of those)?

Thanks.

Wolf Paul