For a detailed GPRS call flow you may refer the book “GPRS in Practice – A Companion to the Specifications ” by McGuiggan P. This is an excellent book to know the internals of GPRS call flow along with the relevant 3GPP specs reference.
If you’re just looking for message flow, you may visit: http://www.eventhelix.com.
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Raj/www.telecomspeaks.com