Hi,

Depends what is the budget involved in the whole design.

If you going to commercialize your security autodialer then you must take into account the possiblities of EMI and other noise.

In high noisy environment, even the transistor will for sure pick up the flase triggering.

I think if you are really serious, you must incorporate in your design the tone decoding technique with a pair of 555 and 567 for long long cables in the extremely noisy envoironment.

Babar