It is set to give me an alert. They are an answering service and they used to alert me when sending my iphone 4 a test message to call clients. Now only alerts me maybe one out of 5 messages. Help!!!
I'd try turning the phone off (hold in the power button, then when the screen changes, slide to power off) and then back on again, full reboot.
If that doesn't work, check to make sure you don't have "Do not disturb" turned on (swipe up from bottom of the screen, make sure the little "moon" icon isn't lit.