I use VOIP (Vonage) and make the settings to call forward all calls after 15 secs of ringing. It will also forward all calls to a phone number of your choice in case the Internet is down. These are permanent settings until you change them.
I have these set to call my cell phone.
Yep. we default program our commercial customers to cell phones, other/ locations and even home lines in the event of disaster.
Cell phones are not our primary advice as the towers rely on electric and cells are quickly overwhelmed by traffic when SHTF.