You’ve probably seen the ads: both AT&T and Verizon have dropped their rates for unlimited voice plans to $69.99 per month. But if you aren’t already on an unlimited plan, you may be paying more for less until you take action.

For example, I was on an $89 voice plan that gave me 1350 minutes a month with rollover. AT&T was going to happily keep charging me $20 per month extra indefinitely. (I effectively had unlimited minutes already — with text and data becoming my dominant means of communication, I had accumulated tens of thousands of rollover minutes.)

$69.99 per month for unlimited voice -- but I had to go online to make the change

I went online to login and make the change to my account. In approximately 90 seconds total, I switched and am now paying $69.99 per month for unlimited voice.

So now my iPhone costs $120 per month ($30 data plan and $20 unlimited texting) before taxes and fees (and apps!).

The change means that AT&T now effectively offers only two voice plans — the only other plan that might make sense is 450 voice minutes for $39.99. So if you’re sure you’re not ever going over on voice minutes, you could save $40 per month. Since AT&T keeps dropping my calls, that’s going to be tempting for me as I start giving out my Google Voice number and using my Google Nexus One phone on T-Mobile more.

Hopefully, dropping voice rates isn’t just the prelude to the carrier’s plans to raise data rates.