Have been in the alarm business before. CellGuard is a PC board cell phone designed to interface with the alarm system. Beware you still got to pay the monthly service fee for the usage (use to be around 9.95/month+usage time+annual system licence fee, but may have been changed) on top of your existing monitoring fee. ATD is now franchised, customer service is really a big concern, most of the time you will need to wait 2 days before you get a call back which is unacceptable because you are talking security!
Regarding cuting power to you home, the alarm system should have a back up battery that should be able to stand by for at least 4 hours. The other concern is if you switch to Voip, you need to disconnect the demarc from Telus & hook up the ATA adaptor to the demarc. Since alarm systems use RJ-31X jack (from demarc phone is first route to alarm systems first, then to other jacks in the house, in case the alarm is tripped, even if you are on the phone the RJ-31X jack is use to cut you off & dial to monitoring station first).
You also need to check that your Voip service is able to dail REALIBLEY to the monitoring station. To test that out, you should enroll in Voip service first, temporary disconnet the Telus line from the demarc (A MUST), wire in the line from ATA adaptor to the demarc. Phone monitoring station first to advise of the testing, then trip the alarm let the systems dail to the monitoring station. Vertify the station got your alarm signal, I would at least make a few test in different time of the days to be sure they got the signal.
If everything goes fine, you may even like to port your existing phone no to the new provider. (A word of caution, some Voip provider is not able to access 800 number in Canada, depends where you are located, your call to the monitoring station may be dailing a local no or a 1-800 number, your alarm company should be able to tell you that over the phone).
Up to this point you can then consider buying UPS for back up in case of power failure.