Hi Raju,
I understand your concern and know that this is the biggest pain for PSTN CUG deployments in India.
The only solution I can think of is to use extension mobility for single line IP phone. Each profile would have either PSTN or CUG access. User would have to manually login and logout and would use appropriate profile depending on whether he.she wants to make a PSTN or a CG call.
As far as the analog phones are concerned there is no other option as far as I know. You'll have to configure it either in PSTN or CUG partition.
Having said this there is a ray of light for us in future. Cisco is corrently working on this requirement for indian customer. In some upcoming callmanager release you would be able to use a single line IP phone for both PSTN and CUG calls abiding by the laid guidelines. This would be trunk implementation of partition instead of line implementation [current].
-> Sushil