Two cell phones that share the same number may sound convenient, since it would essentially be like having a home phone outside the house, but it isn't generally possible. Even if you pay for two separate plans, carriers won't activate the same number on two phones. If you want two separate cell phones, you will have to activate two separate plans and numbers.
Main Phone Number
Your main phone number is provided by your carrier and is assigned only to your phone. If you switch carriers, you may be able to bring the number with you but it will still only be active on one phone at a time. One option for getting their main phone number to ring on another phone is the Google Voice add-on for Sprint phones. If you sign up for it, you get a Google Voice account with the same number as your Sprint number. You can then authorize another cell phone to ring when your Google Voice number -- which is the same as your primary Sprint number -- is phoned.
Another option is to forward calls to the phone you want to ring at a certain time. If you want to share a line so that your spouse can answer calls when you're at work, forward your phone number to her line when you leave for the day and undo the forwarding when you leave work for the day. Since your phone won't ring when you forward calls to another number, it's not an ideal solution if your goal is to have two phones ring.
If you need to share a phone number with someone, consider using an app that has the functionality to make calls and send text messages using Wi-Fi or mobile data networks. Apps like Google Voice, TextPlus and Skype Premium can be signed in on two devices and will update calls and text messages on both active devices. Even though it isn't a phone number issued by your carrier, it will give you and another person the same number on two different devices.
Since your phone number is dependent on your SIM card when you're using unlocked GSM phones, you can share a number by swapping SIM cards. However, only one phone will be active at any given time. Power down both phones and remove the SIM card. In many cases, it's under the battery cover. Place the SIM card into the other phone, power it on and it will be updated to use with the phone number active on that SIM.
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