I was given a replacement from a friend of another Samsung Galaxy S4 from another carrier (AT&T) and wish to put Verizon service onto it since the same SIM card fits. How do I switch the AT&T phone to Verizon?
That device doesn't have the hardware to operate on Verizon's network, but with any compatible unlocked phone from another source you would insert a Verizon sim to activate it.
We want to make sure that you have a functioning device. We cannot guarantee that devices provided through another provider will function properly on our network due to differing network technologies. You can check to see if a device is compatible by following this link: http://www.verizonwireless.com/bring-your-own-device/.
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Older Verizon Wireless and at&t phones cannot be ported between each other's network as Verizon Wireless uses CDMA while at&t uses GSM. I'm not sure that the S4 can support the opposite network. Most newer models can. Check the link and see what it shows.