My Threat worked last month
I called about it last month. Eventually I got put through to customer retention. After debating a bit she counted the number of months left on my newest phone, multiplied that by the 5 lines and that by .61. I don't remember the exact amount, but the credit was around $75. This seemed fair to me. I know this was just a customer good will credit and to make me go away happy, but it will cover the $3.05 per month until the time when I can choose to stay and pay the increased price or leave. By that time I hope that the Sprint network has improved and they still offer unlimited data. I only stay with AT&T right now because I have unlimited data on my line and Sprint network speed is so bad.
Be polite. Be persistent. Use good examples, like "I realize this is only $3/month, but what if it was $30/month for a new fee. Wouldn't you consider that materially changing the contract? They will agree. Well then, why should the amount determine whether or not it is a change in terms". The other thing they like to argue is that it is just a change in billing, ie an accounting change. I said "ok, then I expect the amount above the line to be decreased by the same amount, since this cost has obviously always been there and you have been paying it as part of my contract with the money we agreed upon."
Be polite, it's not the CSR's decision to add the fee to our bill.