Thumb Cellular and MMS

I recently moved to the thumb area of Michigan. A couple of people I know get their cellular service through a local company called Thumb Cellular. I can call and SMS them just fine. If I send them an MMS, they receive it, but if they reply to my MMS or send one to me, I see nothing. Any idea where I could even begin to look to find a solution?

FWIW, I’m using a Pixel 3a XL with the October 5 update and the stock messages app. One of the people I had issues with has a Samsung Android phone and the other has an iPhone. The only commonality seems to be their provider.

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