Option 1: Using a Mobile Phone as SMS Gateway (or a Modem)
For cases in which the Internet is not reachable any more, you can configure notifications using third party software which in turn uses a connected cell phone to send out SMS messages. PRTG does not support this directly, but you can call third party software with the corresponding parameters using a PRTG Custom EXE Notification. For more information, please see How can I send SMS text message notifications via a modem or a mobile phone with PRTG?. The software described on this page can also use a modem (connected to a standard telephone line) to create a connection.
Option 2: Using Multiple Data Lines
Order a second data line for your office/datacenter and run a second copy of PRTG inside your datacenter that uses this data line as its gateway. In most cases the freeware of PRTG is enough to do this!
At the Paessler office we use 5 Internet connections for our data center (our primary professional fiber optic connection, one consumer ADSL connection, one consumer VDSL connection and one wireless connection). If any of them fails we still have the others to send SMS using one of the SMS gateways on the Internet.
Option 3: Monitoring "From the Outside to the Inside"
But please also consider the other "direction": Run a second copy of PRTG outside your office/data center and let it monitor your connection. In most cases the freeware of PRTG is enough to do this!
Consider purchasing a cloud server from "Amazon AWS EC2" or "The RackSpace Cloud" to run this instance.