...in case of misunderstanding...

My '51 board with XR2206 WAS NOT intended to detect the presence of CLIP stream. It was designed simply to detect any signal presented with Bell 202 standard, ie. 1200 hz of MARK, 2400 hz of SPACE with 1200 baud.
So it is not important regarding to phone hook switch position. I need to detect the presence of the stream on-hooked or off-hooked.
An alternative to do the testing (I think) is to make a connection between two telephone units with a modem attached in one side and my '51 board in other side, then I need to make the modem to send the FSK stream.....

Can the modem do the task?

~fhy