Rule 18 (Mark-Room) begins to apply between two boats when the first one enters the zone at a mark, and it says the outside or clear astern boat must provide mark-room.
Note that rule 18 never requires an inside or clear ahead boat to make any kind of hail for mark-room.
There are only two rules in the rulebook that require a hail: 1) Rule 61.1(a) when you must hail ‘Protest’ to another boat, and 2) rule 20 when a boat must hail if she needs room to tack at an obstruction.
So a boat that is required to give mark-room must provide that room whether or not she hears a hail.
However, it can be helpful for the boat entitled to mark-room to make a hail to that effect.
Even though it is not required, a hail can remind the outside boat of her obligation to provide mark-room, and it can help avoid a messy situation where both boats think they may be entitled to mark-room.
Communication between competing boats is often helpful even when it’s not required, especially in tight situations such as mark roundings.
By proactively talking with nearby boats you can often clarify each boat’s rights and avoid risky situations.