JI Net‘s VOIP promotion states calls as cheap as 1 baht, but The Nation says 2 baht/ min. Either way, what you need is a ATA (analog telephone converter) to plug in your home hard phone into. (can use your home cordless if you still have one.)
JI net is selling it for 2,795 THB. Then of course you need a JI broadband connection.
Another option is getting a Skype Wifi phone for about 5,100 baht allowing you more freedom to roam about in your wifi hotspot. Skype Out rates are as low as 0.58 baht / min. You can pay with your Thai Paypal. And Skype to Skype is of course free.