Our church is undergoing a renovation right now, and one of the things that will be put in are wall-mounted sacristy bells. I am not familiar with this. How is this used, considering that the servers are kneeling near the altar, and the bells are apparently on a wall, not within reach (at least in my church).
They are used at the beginning of Mass as a way of announcing the priest entering the sanctuary from the sacristy.
The purpose of the sacristy bell is to signal the people to stand for the procession. It is a more reverent alternative to a wordy introduction by the lector. In many countries (outside North America, sacristy bells are standard in most parishes), the processional hymn is begun while people are still seated, and they stand when the sacristy bell is rung as the procession begins.
I mostly see the sacristy bells used during daily mass when the priest walks right from the sacristy to the Sanctuary, instead of having an entrance hymn (have a spoken antiphon instead), the bell is rung to signal the start of Mass.
I have also seen it used in the Extraordinary form of the Mass, again there is no entrance hymn, and instead the organist is playing, and the bell is rung as the servers emerge from the sacristy, as as sign that everyone should stand, the procession has begun.