Change asterisk IP address

hi all,
I have an issue whereby I’ve changed the ip address of my asterisk pbx, and now all the SIP phones aren’t registering …
I’ve gone into each one and changed the register address to the new one, though after a while it reverts to the old address …
i’m not entirely sure whether there is a config setting on the server that I need to change to make this change?
coiuld someone help?

much obliged,