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I have a p20 pro and when send texts to one number it sometimes send but often says not sent error codes 332 and 500. Also have done sim change. Finally when they say not sent sometimes they have. Additionally, very rarely it has happened on other numbers
If this is just one number where there is a problem receiving your texts messages and your messages are correctly leaving the network for forwarding onto the receiver, the most likely reason will be at the receiving end.
The best way to get this sorted would be to ask the receiver to try the SIM in an alternative phone, but your receiver may find a new SIM solved the problem. If the person you are sending the message to contacts their network Customer Services they will be able to make sure everything is correct at network level.
Further info here: What should I do if I can’t send or receive text messages?