I haven't been able to verify it, but one source (can't find it again) but the telegraphic call of the dispatching office in LIC was "X". Each dispatcher was assigned a section and that would have one circuit of towers and or ticket offices within that section. Sometimes he would have a trainee/poster who would be called a "Copier" who's job was to man the key. However his call would be rarely used, as you would drop your call to get him and he would drop your call if he wanted you.
So, if was working Dunton and wanted to get his attention, I'd key in "MP" and wait for him to send either a go-ahead sign (usually "K") or a "?". Remember they used Continental Code (as opposed to International Morse Code used now) and that's actually harder to pick up than IMC. It was considered faster (or very "ditty") than IMC and once you got familiar with the procedure and the short-cuts then it was reasonably fast, about 20 words per minute if everyone was skilled. And right about that time you would get Carpal Tunnel Syndrome (not understood back then, but they called it Glass Arm). Keep in mind also there was no speakers for hearing it, rather they used Sounders, which was basically a thing like a stapler with no staples going up and down. The interval between it dropping would be a dit, a dah, a long dah or a very long dah. They would try to amplify it by mounting it in a wooden enclosure open to the front and sometimes the agent would place a tin can in it to amplify it or to differentiate it from another sounder nearby. It wasn't easy and not everyone could do it. Lousy or new operators were referred to as Hams, which is where the name came from in Ham Radio.
I'm not sure when they went from Morse to using Magneto phones but probably by the time they moved from LIC to Jamaica. Some locations out East may have used it later as it was tough to run phones out there and Western Union was still busy and still used railroad stations as offices in lightly populated areas and places where there was the military. 204 was the original room number in the new Jamaica office when they first went in from LIC, 204 has bounced back and forth from the 3rd floor to the 2nd as they changed with technology.