SMS Error

We have six cell phones set to receive SMS alerts from our Sensor Gateway. All cell phones receive the test alert but when I test with a real alert by applying water to the water detection cable, all but one cell phone does not receive the alert. Any help on why only one would not receive the alert.

DId you set the alert trigger on sensor status page?

Yep, I have SMS enabled on the Sensor Status page. When I send a test SMS on the UIN and PIN page, the "Sending SMS" says status "failed".