1) Please replace current phone number with my new phone number.I have a new phone number. Please update your records to reflect that my current number is [number].
2) Send this message "to my phone number" or "on my phone number" ?
And if that had already been established, wouldn't you add "the" before it instead? I find "Please replace current phone number" quite unnatural. It sounds like something that might appear in a list of instructions but I can't imagine saying it to anyone.