A friend had an iPhone 6 and received a newer Samsung Galaxy S20 phone. He moved the SIM card from the iPhone to the Samsung phone. He had trouble receiving text messages.
He removed the SIM card and put it back in the iPhone and followed instructions to delete all messages on the SIM card. Now, in the Samsung phone, it receives text messages fine from most people, but certain iPhone user texts are not received.
Nowadays, mainly in the US that I have seen it, sim cards are one way or another link to a device imei. He probably just need to call the carrier and they will update it. Another thing, his friends might set the messages to only be sent as imessage but I am probably 90% wrong, I haven't touch and iphone in years. Try to test it with another iPhone.
iPhone users send text messages using the iOS app "Messages" by Apple. This app can send SMS and iMessages, the latter is an Apple proprietary messaging system.
If you move from an iPhone to a non-iPhone then the other iPhone users will still send you iMessages instead of SMS as the iMessage system still knows your phone number and associates your old iPhone to it. It doesn't know that you have moved to a phone not capable to receive iMessage.
Therefore you have to inform Apple/the iMessage system that you can no longer receive iMessages: you have to deregister your phone number at Apple.
Apple provides a web site for deregistering your phone number in the iMessage system: https://selfsolve.apple.com/deregister-imessage/
