Can't text from iPhone

I recently bought an iPhone 7. I can call from it, I receive calls and I receive texts but whenever I try to text someone, the message says not delivered. Most of my contacts don't have iPhones so I can't use I message. The speech bubble is green (which I think means normal sms) but I don't know why they're not leaving.

Get iPhone X. IPhone 7 is outdated you dummy

Your phone is not fully activated on the network. Call your cellular service provider.

You should contact your carrier about that issue.

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