I was really concerned about what was planned for this device since there are many improper uses for such a device and it is very, very unusual to need any kind of phone triggered device. I still struggle with this a bit. Knowing that there are inappropriate uses to the technology I really don't know why you're so angry that I've had a concern about the final application.
Throughout the thread you have been secretive about what you planned to do with such a device. If this was a straightforward application you probably would have told us all about it by now.
If this is related to your attempt to capture callers numbers, then I suspect you may be dissappointed - it's easy to withold a number from nuisance calls and you'd have to involve the authorities to capture the real number.
You've already had a load of suggestions from other posters.
I've raised my concerns, you've half explained why you are pursuing this - it's time to move on.
If this is really required to aid a family member, concentrate on them and consider if this route is really the best form of action.