Interfacing alarm using Contact-ID

[etolocka]

Hi

I´m working on a interface to a DSC alarm using the contac-id protocol. I need to emulate a telephone line and generate dial-tone and handshake signals. I have a prototype working, but sometimes the alarm misses the signals.
The line emulation is done with a LM317 connected as current source from +12V and the tones are generated by the microcontroller´s timer and filtered with a passive low-pass filter.

Anyone has experiencie with this? I need some advice on the topic.

Regards.