Did you restart your phone after changing carriers? Or clear cache in the carrier services app? Those may help, but don't fret about it. There are behind the scenes settings and apps that "tell" the phone which carrier it's on. They are often slow to update when you change carriers, but eventually they will. Shouldn't take more than one monthly cycle so if you just changed, it should resolve itself in early August.
You could try to force it by turning off your phone, removing the sim card, restarting the phone without the sim, then turning off and restarting with the sim in place. That may clear old carrier settings from the phone. Again, don't fret if this doesn't immediately help. If the problem persists more than 3-4 weeks then contact tech support.
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