Why don't people receive my texts with iPhone?

I have a iPhone and have done for years, but for the few months people have been saying they don't receive my texts. They do sometimes but 50% of the time they don't. Some are iPhone users and some aren't. It's a massive problem and I'm going to ditch Apple completely unless I find a solution. I've read things about iMessage not working so I've turned it off. But it's just my text messages don't get through. I get texts asking why I don't reply! Please someone help or tell me it's not just me, if it's an apple problem then they are gone, if it's my phone company then they are gone. I need to ditch whatever the problem is.

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