Problem with SIM900 multi connection

Hi
I want to set Local Port for my sim900 module in multi connection mode(i define 3 server for GPRS connection).
but i have problem!!
after 2 days continuously work in see ERROR in AT+CIFSR command!
in my simcard APN(Access point network) is active.
any ane can help me to solve this problem??
should i active PDP context for multi connection??
thank for you attention