could not activate cellular data network on iphone 6 plus

I'm having the same issues. AT&T blames it on a "back logged switch". The filed a trouble ticket and advised me they would contact me in 5 days.

I have the same issue starting from yesterday, at&t said it would be Apple's issue, but my friend's go phone sim card works perfectly well on my new iphone 6. All the help I got from both at&t and Apple was just asking me to restart and some basic stuff.

waiting for the answer......

Do you have an AT&T Store near you? If so, go there and ask them if they could provision you another nanoSIM to try in your new phone. Could you supply me with the first 14 digits of your SIM/ICCID #. They are 20 digits long so providing the first 14 is of no security concern. This mainly identifies the SIM card manufacturer and batch #.

AT&T Store Locator: http://www.att.com/maps/store-locator.html

Ok, glad you got it resolved.

Thank you,

I'm having the same issue with my iphone 6. Called AT&T and was told it was a network issue with prepaid and that they have no resolution for it. How did you get ahold of of the person that helped you to get your phone to work?

Could you supply me with the first 14 digits of your SIM/ICCID #. They are 20 digits long so providing the first 14 is of no security concern. This mainly identifies the SIM card manufacturer and batch #. You can find in Settings, General, About.

I posted the solution that worked for me here on Apple's Community Site:

Iphone 6 cellular data not working on prepay go phone service

Good Luck!

Thanks for the info, it worked! When I was talking to the AT&T rep it sounded like he had no clue about what I told him to do but he was cool and did exactly what you said to do on your post. In the end he was just as surprised as me that it worked. And for the record, my Phone was an AT&T phone that I bought at BestBuy. So I would assume that this solution will work on any GSM iPhone 6 that you want to put on an AT&T GoPhone plan.

Gaps solution worked for my Verizon iPhone 6 that I'm running on GoPhone as well (Thanks!). You will need a new SIM card at some point, however, if you want VoLTE when AT&T rolls it out next year.

Are you getting LTE on that phone? From what I understand the LTE does not work when using a Verizon phone on an AT&T(GSM) network.

I am absolutely getting LTE. From the iPhone 5S on, the GSM and CDMA phones had identical GSM radios ostensibly for overseas use (in the case of the Verizon phones). Verizon tells you they're only unlocked for overseas so that you won't leave their service for a domestic GSM. Actually, all of the 6's have identical receivers, however the CDMA antennae are disabled on the T-Mobile and AT&T versions... which doesn't matter anyway because Verizon and Sprint won't activate devices that did not originate on their networks though theoretically you could run on Verizon LTE with a GSM only phone as GSM is required for LTE (thus the SIM card in later model Verizon phones).

The Verizon model is the best of everything because it is completely unlocked for any carrier (except Sprint, but who cares, right?).

Edit: forgot to mention... Verizon is required to make all of their LTE phones GSM-unlocked as per their agreement with the FCC when they were acquiring LTE technology.