AT&T has made no official announcement of this price hike as of this writing, but trustworthy outlets have reportedly confirmed the policy modification with AT&T representatives and customers are already starting to receive notifications.
The $5/month price increase only affects mobile customers who have held onto unlimited iPhone data plans after AT&T stopped selling them to new subscribers back in 2010.
This price increase will increase the unlimited data portion of the phone bill from $35/month to $40/month.
A network management threshold still applies, meaning customers who use more than 22GB of data in a month may be subject to de-prioritization while on congested towers.
Grandfathered AT&T unlimited plans also do not allow for mobile hotspot use or tethering - data is on device only. 'Jailbreaking' a device in order to allow for tethering while on a grandfathered AT&T unlimited plan has a history of the plan being revoked.
Starting last year, AT&T brought back their unlimited smartphone plans, but only for DirecTV subscribers. DirecTV (owned by AT&T) customers can obtain an unlimited smartphone, tablet or connected car plan for $40 month (same price as the grandfathered plans are going to). These DirecTV associated unlimited plans are not incurring a price increase at this time.