AT&T SIM card for GSM dialer
I want to get a GSM cellular dialer for my alarm system but I am not sure what to get. The seller says the dialer is compatible with AT&T, Cingular, T-Mobile, and Beyond Wireless GSM cards. I know there is a AT&T tower close to my home but I am not sure about the others, my cell phones are with Verizon so I have not had to use a SIM card yet. I need the lowest cost way to get a card that the minutes wont expire right away and I don't need the 1000 minute card to get one year of service, I would only need enough minutes for the alarm to call out in an emergency and monthly test calls, any suggestions on what I should do? Sorry for the lame questions but I just haven't had to deal with SIM cards before.
Edit: I just checked and T-Mobile has coverage in my area as well.