I have an AT&T iPhone 5s for use in the U.S.A. I will be travelling to Europe where I will be using a T-Mobile iPhone 6 that has their Simple Choice Plan (unlimited text and data with .20 per minute calling). If I turn on call forwarding from my AT&T iPhone 5s to my T-Mobile iPhone 6 while in Europe will I incur any charges from AT&T?
That's a good one, I am not 100% sure, but my answer to you is no because the destination of the phone call is a tmobile number, and tmobile is registered within U.S., so I would say you can make a good valid argument about that.
I would be very careful trying to use that arguement because T-Mobile is also registered in Germany, and when theT-Mobile phone is roaming in Germany, it wcould be then show as a Germany phone. IIRC, T-Mobile is registered in most of Europe, so I would suspect that you would encounter international call rate if you forward your phone to a number that is roaming internationally.
One thing I had noticed before when using call forwarding here in the US. I was forwarding from one AT&T cell to a second, and since AT&T had free calling from mobile-to-mobile, I didn't expect any charges against the receiving phone, but in fact the phone being forwarded never showed any activity (it was off in a desk drawer) and the receiving phone was charged for all incoming calls, and actually showed callerid as if the calls were direct dialed to it. Since that was AT&T to AT&T forwarding it might be "special" but based on that experience, I would bet that calls forwarded to an off-carrier phone, you would be billed for the calls that are forwarded, especially those forwarded internationally.
I found this under "Important to know when you use Call Forwarding:
Forwarded calls are billed airtime for the length of the call. Long distance and/or roaming charges may be incurred.
Long distance charges apply if the call is forwarded to a number outside your designated home calling area.
With some AT&T Wireless plans, Call Forwarding minutes don't apply to your included minutes.
You can continue to make outgoing calls from your wireless device. Incoming calls will be forwarded to the designated number. Call Forwarding overrides your wireless voicemail.
When the number to which your calls are being forwarded is busy, callers to your number will receive a busy signal.
Calls can't be forwarded to an international number.
What's interesting is that "Calls can't be forwarded to an international number" would lead one to belive that there would be no international roaming charges from AT&T. So currently when I have my AT&T phone call forward to my T-Mobile phone that is a mobile to mobile phone call. Both phones are in the same area code. At most I can see AT&T using my anytime minutes of which I have plenty. But after the call is forwarded to the T-Mobile number then AT&T is done.
I spoke to an AT&T customer service rep who told me at most it may use Anytime Minutes but the call is nothing more than a local carrier to local carrier or mobile to mobile call. But who knows what arcane rule may be invoked in actual practice? I guess I'll have to try and find out.