Is it possible to unblock phone using imei

You can't bypass activation lock, that is the point of it.

Why can't you enter the id and password of the account that it's locked to ?

If you've forgotten your id have you tried : If you forgot your Apple ID - Apple Support ?

Or, and if you were the original purchaser of the phone (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (http://www.apple.com/support/itunes/ww) and see if they can remove the phone from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it

If this is a used device, you will need to contact the seller to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless.

Activation Lock - Turn off Find My iPhone