Interesting, do you know if the folks not receiving messages are on one carrier? I wonder if there is an issue where your phones are still trying to send the message through ATT for ATT users?
Have you tried resetting the network settings in the phones?
Few basic trouble shooting tips:
https://community.visible.com/t5/Plans-Service/Quick-solutions-to-common-wireless-service-problems/t...
If a post has solved your problem, please mark it as an acceptable solution, it helps others that may have experienced a similar issue get a quick resolution.
