I don´t think you can, all i get is those annoying ringthai and tvpool messages.
upto 12 months before you lose the number, as long as you leave credit on it.
When you top up (with 12call) it should tell you the date when that top up will expire, which should be in 12 months time.
I refilled last time april 3rd, i still have credits on, but it expires Oct 15th 2007.
Hmm, maybe thats wrong. Its says "you have 130 bht, valid until 15/10/07" Thats the credits valid, but it doesnt´t say how long the card is ok.