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you will be able to make phone calls from the iPhone 3G when you leave the store, but you may still receive incoming calls on your old cellphone for 6-8 hours if you are porting your number. i found this out this morning at an apple retail store.
 
... or do we have to finish it at home? anyone have any clue?

Currently, with the information available, it is believed that the devices must be 'unbricked' in-store, attached to a contract,
and then you finish registering the device, through iTunes 7.7, when you get home.

Whether you can make calls, before finishing the registration process, is doubtful.

[No longer relevant - see below]

R-Fly
 
Currently, with the information available, it is believed that the devices must be 'unbricked' in-store, attached to a contract,
and then you finish registering the device, through iTunes 7.7, when you get home.

Whether you can make calls, before finishing the registration process, is doubtful.

R-Fly

Based on the front page of MR:
— best of all — have your new iPhone 3G ready to make calls, browse the web, and receive email right on the spot.
 
but it does say on the o2 website that you purchase your phone instore pick your tariff, then you have to activate your phone using iTunes?

Check Here
 
you may still receive incoming calls on your old cellphone for 6-8 hours if you are porting your number. i found this out this morning at an apple retail store.

It probably depends on luck of the draw. I got mine on Christmas day and activated it then. I ported my number from Verizon and I was making/receiving calls within 5 minutes of activation. I was quite amazed at the speed and the beauty of the automation.
 
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