I am not sure how long this has been going on, but I have been using this for the past few weeks and have found it very useful.

AIS has a promotion (on a prepay plan) where you pay 399 baht and then for 1 month you get unlimited free calling from 6 AM to 4 PM to any phone numbers within Thailand, mobile or land. Great for those like me who need to do a lot of daytime calling. They also offer 10 PM to 8 AM instead, if you prefer these hours.

Outside of your plan hours you pay 5 baht for the 1st minute and 2 baht for each additional minute. The plan is called Kui Mai Aun.

Enjoy

A couple of hours after posting this I found out that DTAC has a similar, but better promotion now as well. It only costs 299 baht per month and it runs from 5 AM to 5 PM for the unlimited calls.

And finally Orange has a different promotion where I believe all calls are 25 satang for the first minute and 1.50 each additional 24 hours a day, with no extra monthly fee. But you better check on the details yourself before you get this as it seems to me that it should be that the first minute is more expensive and the additional only 25 satang.

Take your pick. AIS still does seem to have the best coverage and signal strength however, with Orange being the worst. Though I hear from friends using this that it is improving.

It seems that the prepay plans are better these days than the monthly billing plans, for most kinds of usage anyway, from what I can see.