Input credit - please guide

Do u have centralized registration?

When services are rendered at two or more places centralised registration will be obtained.I think you might have obtained centralised registration at the time of obtaining registration .

You can take input of service tax paid on rent of your Delhi office.

You can also take input of service tax  paid on mobile phones provided the mobile phones are used for the purpose of output service.

Do i need any input credit registration in delhi service tax department or simple amending to centralize registration is enough?

In my opinion if you amend the existing certificate it is enough as service tax is a central subject and there is no need to obtain any registration from delhi st department