From The Code:
Receiver readiness. The receiver shall play to the reasonable pace of the server. The receiver should make no effort to return a serve when the receiver is not ready. If a player attempts to return a serve (even if it is a “quick” serve), then the receiver (or receiving team) is presumed to be ready.
And from the ITF Rules of Tennis:
21. WHEN TO SERVE & RECEIVE
The server shall not serve until the receiver is ready. However, the receiver shall play
to the reasonable pace of the server and shall be ready to receive within a reasonable
time of the server being ready.
A receiver who attempts to return the service shall be considered as being ready. If it
is demonstrated that the receiver is not ready, the service cannot be called a fault.
These do not discuss becoming unready, but I think it is reasonable (and there are other rules dealing with hinderances) that if an outside interference occurs the player can become unready. And certainly if the score is not called until the player is ready, they have to be able to become unready in order to dispute it before the point begins.
I suppose technically if things have become contentious, the receiver should hold their hand up until the score is called.