GPRS Modem Interfacing with PIC18F8722 Problem??

ok.. I think you first need to understand the dormant state of modem. In dormancy GPRS connection stays alive but its not kind of active which means there is no actual data being transmitted or received at that time. So when you come out of dormancy mode your IP stays active (not changed unlike new GPRS connection) and you can resume data transfer again.

But not every GSM modem performs proper dormancy.. you will see your GSM device will show GPRS is on but you wont be able to send/recevie anything. Thats what the problem is. Dormant mode get active when you do not use GSM modem for a long time. usually its more than 15 minutes.. So if your modem is not communicating more than 15 or 20 mins then modem goes dormant.