New iPhone and new sim messages not sending?

They send 90% of the way then stop and fail. I've topped up 20 pounds by ringing up and I got a conformation text but I can't text "dolphin" to 450 it fails
I got the do phil package and it tells me on the package to text that to that number
I don't understand, how can I get my messages to semd

Call your carrier's customer service center or try to ask apple for a fix.

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