Problem:

The customer is unable to receive inbound calls. 

Solution:

This is related to the IP Handset (IP Centrex), or the Gateway Device (IBL, PRI) ability to register to Thinktels network. There are a few possibilities:
1. Power Outages
2. Loss of Internet Connectivity 
3. Router / Firewall security options are blocking VOIP traffic.

Note: Calls should still route to Auto Attendant and/or VM. A customer can also choose an emergency routing failover(SureCall) i.e. route to a cellular phone.