Different mobile phone companies
I was told that my iphone used with ATT would not work with Spectrum. Did they feed me “story” or are iphones fitted differently for each specific carrier?
iPhone SE, iOS 15
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Many are "carrier locked" and stay that way until the carrier flags it for unlocking. My wife had an AT&T carrier locked phone and eventually got it unlocked once the (subsidized phone) contract terms were over. If a carrier-locked phone is purchased full retail without a contract, then the customer can typically request the lock be removed immediately. Some companies have broad policies, like Verizon which has their locks automatically removed after 60 days after activation, as long as the customer's account is in good standing, but that's the case if it's purchased full retail or with a contract that subsidizes the cost of the phone.
What a carrier lock means is that it will be tied to the service of that specific carrier until it's unlocked. Apple sells "SIM free" devices that can be freely used with any carrier. Some iPhones will be locked to the first carrier for which a SIM card is used.
How to unlock your iPhone for use with a different carrier - Apple Support