Try a different carrier without porting number but appearing to be old number

[zucchiniknob]

I currently have an iPhone 5S with AT&T. I would like to try T-Mobile without porting my current number but in such a way as my friends and contacts see my current AT&T number, not the new T-Mobile number. Having tried it, it I like T-Mobile, then I will port my old number.

But in the meantime, I don't want to confuse friends and contacts that my number has changed then possibly changing back to the old number. This should really be a thing to help you evaluate a new service.

I think that I can forward calls to my old number to the new number using Settings->Phone->Call Forwarding.

But how do I make it so that when I call or send a text (or iMessage) that it appears to be coming from my old number?

 

[Quis89]

If you're phone is unlocked, why not just switch over to T Mobile and get a new SIM card. If it doesn't work, switch back. Unless your grandfathered into an old plan with AT&T that you don't want to lose, I don't see the disadvantage here. Using the same phone will save you from spending money on new equipment and if the phone is unlocked I believe it will work with T Mobile as well.

 

[BreakingKayfabe]

For an iMessage, it's simple. Your old number should still be registered under Settings - Messages - Send & Receive

To mask your new number to appear as your old one in caller ID is a different story. I don't know how to do that.

Edit: spelling error

 

[zucchiniknob]

The phone will be unlocked soon. I just received the OK and procedure from AT&T to unlock. Nothing grandfathered. My line is the main line in AT&T Mobile Share Value plan with 4 lines. I guess it's not a problem to make one of the three remaining lines the main line. If I decide I like T-Mobile, I'll move my wife's line but the kids are on their own. The new plan is a plan for seniors (55+) and only allows two lines.

 

[zucchiniknob]

Phone is unlocked. AT&T did something on their end, and then I backed up to iTunes (not cloud), reset all content and settings, and restored the phone from the backup. Somewhere in the process, iTunes showed a message that the phone is unlocked.