Help needed for sms notification

Hello guys.

Here is the scenario: RB951 with pppoe-out1 (main) + local USB 3G modem (backup). I have set route for the 3G with options: checkgateway=ping distance=10 scope=30 targetscope=10
The backup is working fine. When the main uplink (PPPoE) goes down all the traffic has routed through the 3G .

The task: I wanna RB to notify me with SMS when there is no Internet through the PPPoE connection (or when 3G goes active). I have tried the Netwatch tool, but with it there is no possibility to specify an interface through you are checking connectivity to some host.

Thank you in advance.