Thank you for this. To give you some idea, I have about 90 contacts in my address book who reside in foreign countries and the specific individuals change from time to time (new people in the same positions). I would need to contact all of them, determine their carrier, compare it against Verizon's list of carriers, then have a text/no text list, then refer to that list before texting. But even then, Verizon's list, I have found, is not reliable and they have advised me that it changes regularly (probably why the list is not reliable).
I have occasional, but not continuous need to text some of these people. For example two, an Australian and a Dane were recently traveling in the US and I wished to contact them. I could have phoned them, but that would have caused long distance charges for both me and for them, whereas texting would be around $0.50 per message for each of us. I thus texted them with key information on meeting dates and times. When I didn't hear from them for two days, I became suspicious and ultimately called to learn that they had received no text messages.
For those with fewer overseas contacts and an occasional desire to text, I would say that the best option is to text them nonessential messages and see if they work. If you take the time to find out their carrier, you would have to contact them anyway and then compare to the list which again, seems to be unreliable. The key message is that the system is unreliable and the reason for this. There are definitely workarounds.