CISCO IP phone 7940 Series

Do you have provisioning setup on your call manager?  If you do this may be preventing changes to the phone.

If you need a static IP you can put a reservation in your DHCP so the phone always gets the same IP, you can also set other DHCP options with a reservation.

Well i dnot setup this my self but how can i check for the provisioning and remove them how can i reset this phone

You do not want to remove provisioning because then you would need to manually configure every single phone.

What is the ultimate goal with this phone?

Is to assign Ip address manually

But why do you need to assign IP manually?

If all you need is an IP that does not change your better option is to set a DHCP reservation, in general beyond servers/ network equipment you should not have any devices with static IP on your network.

The problem is that the phone have a IP address that is assign to it already but its not in our network so i am trying to remove this IP address form the phone and not able

Where is it getting it's IP from?  Is this a new phone or did you buy used?

If you purchased used I would advise a full factory reset of the phone.

this  phone is used but it has been giving to use from a other site but this IP is a other network

This information help in the solving of the problem