Debug problem in 18f452 communication with SIM300

Hello i am using a customer battery charger for my work it's been developed by some electronic engg but the problem i am getting is most of the time the the MCU is not able to communicate with the SIM300 and also many times the SIM300 module is getting burnt(the tx,rx pin does not work) i need some one to fix this and suggest hardware changes(i need direct communication with the module without MAX232) i am using a buck converter to supply 4v to the micro controler also need coding for other works like it has temperature sensor, SMS based device on off, and battery voltage detection etc. i have the code for this just need fixups NOTE: this is a patented design and you must not disclose any of the code/design to anyone apart form our company staff