Why did my iphones carrier change?

I'm with telstra, and now my phone says carrier as optus. My phone is locked to telstra but i thought it wasn't and ordered a tpg sim which i put in it. I can't call anyone but my carrier changed?

I don't really understand what is happening here. It's strange how your carrier has "randomly" changed. Have you checked the sim card? What I recommend you do is ring Telstra and tell them what is happening.

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