Hi Bipin Jaiswal,
thanks for the report.
Have you verified correct configuration of OsmoSGSN, with a different phone? (It sounds a bit like some setting might mismatch, like PLMN...)
It would be excellent if you could create a bug report on osmocom.org, and attach network trace pcaps and full logging to that report. Also explain whether it is always reproducable or sporadic. Thanks!
Please look into this issue or let us know if we have fix for this issue.
This is an Open Source community that lives by contribution.
If you require timely support, you can book it from a professional support vendor (e.g. my employer), which then also contributes back to the Osmocom community.
~N