SOLVED.
This happened to tướng bủ, the exact same message, because the person I was trying to tướng text was using Straight Talk Wireless, and his trương mục had lập cập out of minutes/money. Straight Talk is a prepaid wireless service that uses the Verizon network. When the trương mục runs dry, Verizon blocks the phone. You can't send to tướng it. They can't receive anything.
They can't điện thoại tư vấn or text you to tướng tell you their phone is dead... because their phone is dead.
In my case, I was able to tướng điện thoại tư vấn the Straight Talk phone number (877-430-2355), choose Option #6 for tech tư vấn, enter my friend's phone number, and refill their trương mục using a Service PIN from the back of a Straight Talk "No Contract 30-day Plan" refill thẻ. These are available at major stores in the gift thẻ area. You can use a credit thẻ too if you go to tướng straighttalk.com and tự the refill there. You just need to tướng know their number.
Once I refilled his trương mục with minutes, I could text him immediately.
There are other services besides Straight Talk that use the Verizon network, sánh make sure you know what service they're using.
Hopefully this helps someone.