Why am I randomly not receiving texts?

A few months ago it came to my attention that I was not receiving certain messages from what seemed like a single person (they showed me their phone proving they in fact had sent me these messages).  Then it happened with another friend. Both these individuals have iPhones and are on T-Mobile. I'm an Android user.

It is seemingly completely random, sometimes even in the middle of a conversation, selective texts sent seconds or minutes apart will not come through.

So far I have tried:

• Changing to the stock texting app (I normally use Mood)
• Clearing the text app cache
• uninstalling and reinstalling Mood
• changing the SIM
• T-Mobile advised me to do some kind of reset (which all it did was make me have to put in all my WiFi passwords again)
• full factory reset
• Confirmed APN settings
• Bought another damn phone.

Annoying that I could never figure out wth was happening, but no matter, it seemed like things were working. Well, the problem has returned, and it's very difficult to troubleshoot or know when it's happening because it's not like there's an error message. I don't find out until days or weeks later, and I imagine sometimes never at all. People think I'm just ignoring them.

I have also had issues where texts come in very delayed, I'm talking anywhere from like a half hour to 24 hours later.

These are just regular old sms texts, usually brief ones too, not group chats media etc., if that matters.

Is it possible that that I am momentarily losing service when these messages are being sent and they just kind of disappear into the void? lol

Solutions?