Is your location an area of weak connection to the network? Or perhaps theirs?

Sounds like the network is not recognizing that the message is not getting delivered to the phone so it keeps attempting to send it until it gets an acknowledgement of delivery. Not sure if that is an issue that would be on your side or theirs.

Try reprogramming the phone to the Verizon Wireless network. Dial *228, option 1.

Your friends should try that as well if they have a 3G CDMA only phone. 4G LTE devices don't use that dial code to reconnect to the network.