Go to the "execute command" section and enter this:
Sendsms yourcellnumber "this is a test from my wr21"
When you get the message, you'll know the number of the sim in the router.
Now, in the network-interfaces-mobile-sms tab:
Poll sms input every 1 minutes, set that,
Give the command priority HIGH,
Add you cell number to the list,
Apply, save to startup config.
Now, in your phone sms the following to the router:
In the event log you will see it arrive and execute.
You should get a response.
If you want to reboot, you send "reboot" m