You can reserve a DHCP address for a phone but I don’t think you can set a fixed IP on a IP phone (at least the Cisco ones I have worked with). They don’t have any means of keeping the IP address in NVRAM since they rely on POE for power. There are also other parameters that DHCP offers such as the TFTP server for the phone config, etc.
****There is one way that you can try; When you go to the properties of the scope, Set the lease duration to unlimited. This might act as a workaround *****