If you have an older unlimited plan, prepare to pay at least $10 more per month if you have a single line, or $20 per month if you have multiple lines.
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AT&T Hikes Prices on Older Unlimited Plans, Starting With Your August Bill – CNET
If you have an older unlimited plan, prepare to pay at least $10 more per month if you have a single line, or $20 per month if you have multiple lines.
source