Can I make phone calls with VoIP if I lose power?

Losing power can cause your Internet connection to go down.  If this happens you will not be able to make calls unless you are equipped with a backup UPS, which could power the connection for a limited short duration (enough time to finish calls).  One benefit to using SIP Trunks or Hosted PBX with DB Networks is that if you lose power or Internet connectivity due to the carrier being down, you can automatically have a failover option in place that will immediately start forwarding calls to a number specified by you (cell number, etc.).