I have a MotoZ force with os software updated to Jan 24, 2019. Two separate occasions I tried to create a MMS txt group with two people, one with an iphone also a Verizon customer, the other with a wally world straight talk.The person with the straight talk was listed on my phone as a participant in the group and was able to send and receive txts in the group. Later they got a notice with a random phone number with a Mexican area code 588 one could not recall what it was about the other would not accept whatever it requested. That individual, while listed as a participant no longer received or could send. The person with the iphone did not receive the message and was able to continue conversation. I left the original group as a participant, the individual with the iphone got a notice I had left the group, and that she was made the admin. I was unable to find anyway to return to that group.
In trying to resolve this, I created a new group, going to Enhanced group upper right menu, to give it a name. This time the individual with the Verizon iphone got the random Mexican area code number notice. The notice is a request to add the number as a new contact. It was not accepted, opened, group has been deleted. I have a group text set up with the same participants but no "enhanced" name, it works with the Verizon participant but not with the Straight talk participant.
I believe I saw Message+ was not not a Verizon app, so is not a Verizon issue to solve. Can you please tell me which TXT app,service, whatever I should be using to secure a reliable, secure connection with a group of people, who are not all Verizon customers and who might not have a smart phone but a simple phone with txt capability?