the best way to siam enlistment once and for all, is be very religious, declare u're a tangki in the temple, or u're a religious worker...... then most likely they not interested to enlist u.
I don't think the Tangki thing works either. I've a friend who is a tangki still kena call back. I'm can't be sure if he'd declared this but I don't see any reason for him not to if he can siam reservist