I want to know whether is it possible to deploy voice setup in 5-10 branches with 2900 series router as a call manager present in a hub location and branches connected like spokes to it.
I m a bit confused about the subnet to be used on the IP Phones? IP phones should be in same subnet or different? How will the phones register on the call manager? And do I need PVDM card...?
No you don't need to VPN in to get a remote phone to register. You just want the phone top be routeable to the IP CME is running on. If you wanted to, I suppose you can run CME on a public IP address, if you wanted, as long as you turn auto registration off.
The SPA 525 works with the UC500 platform and has VPN built into the phone if you wanted to go with that CME platform. Otherwise, if you wanted VPN, you'd need a remote acces router doing VPN in front of the phone.