Power Companies usually have a phone # to ring when there is a power outage. This number is a general one for the area and not specific to the company you buy from. It should be in your local phone book. For example I buy from nPower but the phone # to ring is to United Utilities who deal with emergencies. If the outage is a big one you may get a recorded message explaining the problem but beware because there can be more than one outage and if yours is only a local one it may not have been reported. They usually give an alternative # to ring in these cases and it is wise to phone this # or you may be off for hours before the fault is discovered.
Of course you need a standard phone in these cases. BT recommend having such a phone to hand since mains powered phones are dead during an outage.