I need to put the sim from my iphone 5 into an old 3gs. I bought a sim converter. Do I have to call ATT for anything? I have the old unlimited plan and don't want to loose it.
You might want to call them and give ATT you phone IMEI and possible change you plan.
Having the wrong IMEI can cause problems with voicemail
You can tape it in, too. No need to contact AT&T or anything, you're good to go.
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It'll register automatically.
Source: I've had 4 phones in 4 months.