Let’s assume following scenario: 

• Module is connected to the GPRS.
• DHCP server gives an address with an "lease time" of let say 45 min.
• Ping and other services work fine.
• After some time which is smaller than lease time (i.e. 20 min) module is not able to use the GPRS and ping the server that they could initially ping.
• If the connection is closed from modem side and reopened, everything work fine again.

  The assumption is that MNO cleans and disconnects the inactive clients aggressively and assigns the IP to other clients. The only way to detect this is use any means of transport layer (PING or ARPING) but this is sometimes not allowed by customers as it causes additional costs. If we assume that closing the PDP context will also lead to loss of IP address, a solution for this situation might be periodic poling of the AT commands that give IP address assigned to the module. Following commands can be used on: 

• PLS8 ->  AT+CGCONTRDP=1 ( if AT^SWWAN=1,1 was used to activate CDC-ECM port ).
• PHS8 -> AT+CGPADDR=17   

 After network initiated PDP context deactivation, both commands shall return just OK