The A-56i is a GSM phone and the I50 is an iDEN phone. They operate using two entirely different technologies on two entirely different carriers. Naturally, the sim card from Cingular will not work in your phone from Boost. No way, no how.
I have a Siemens A-56 (w/Cingular) that I dropped one too many times...so I tried putting my sim card in a Motorola i50sx (Boost Mobile), and I get a message that reads, "...please enter special code..."
Do I have to contact Boost to have them unlock it, or is there some other way to do it?