When power is lost and devices restart, the phones usually come back up faster than modems or routers or on site PBXs. Since the modem and/or router (or PBX) are not ready, the phones fail to register when they come up from restart.

The best thing to do is to turn on devices in order so as other devices come up, the devices they require to register are already up.

The best practice is to start from the outside and work your way in.

• Start by power cycling the modem. Once it is up, then go to the next step.
• Next power cycle the router. Once it is up, then go to the next step.
• Next power cycle the phones. (if you have an onsite PBX, power cycle that before the phones).