AT&T is joining the ranks of wireless carriers jacking up the prices of its older plans. The carrier has begun informing users, including with a support page on its website, that starting with their August bill it plans to raise rates by $10 per month for those with just a single line of service or$20 per month if you have multiple lines. According to the support page, the carrier says that the price increase will "affect most of our older unlimited plans" including: AT&T Unlimited & More Premium AT&T Unli ...