When a train wishes to call the RTC, they must key the tower - send three steady tones on the appropriate frequency to a lineside base station (or tower). Portable radios, such as the ones carried by work crews, have a keypad on them that produce codes similar to touch-tones on a telephone. To call RTC using a portable radio, the operator transmits a code, such as *5480# on the calling frequency (This particular code is used to call RTC on CN's Halton subdivision - I'm not too sure about other areas) . Keying the tower activates the radio in the RTC's office (response to the hail consists of three 'ringing' tones on the channel). 

After receiving the call, the dispatcher comes onto the frequency and identifies themselves ("CN YA Toronto, Over" or "CP Galt Sub RTC, over").

Scenario #2 - RTC calling Train

All trains monitor a specific road frequency. CN trains, travelling anywhere on the network, monitor 160.415 MHz. CP trains monitor different frequencies depending on what subdivision they're on. The frequencies are listed in the Frequencies section of this site.

The dispatcher's radio can select specific towers along the subdivision, normally selecting whichever tower is closest to the train they are calling. 
Sometimes you can hear the trains ask the dispatcher to switch to a different tower if the original one isn't coming in clear.

If RTC wants to talk to a train, the dispatcher comes onto the road frequency and asks for the train by number. "633, this is YA." or something similar. If it is a long conversation, the dispatcher might ask the train to switch over to another frequency to free up the main channel. "Switch over to eight" or something similar. 

Scenario #3 - Train calling Train

There is no special protocol here - passing train crews often just jump onto the radio to wish each other good day, to let them know that the visual inspection is okay "633 west okay on the north side", or just to confirm the tee-time for the foursome this weekend.

Scenario #4 - Conductor calling locomotive 

Again, no special protocol here. The conductor often has to leave the train to perform switching operations, lift/set cars or carry out lineside repairs. He/she is in contact with the engineer via two-way radio - usually transmitting on the main calling frequency. All transmissions are usually preceded with the train number, to avoid confusion and disaster if another crew is switching nearby (in case the engineer hears an 'all clear to reverse' from a nearby crew, and thinks its from his conductor who is underneath the train repairing a faulty trainline). Usually you will hear the conductor informing the engineer that its safe to back up, move forward, and how far away the rear of the train is to the cars they are picking up (usually expressed in car lengths, and then feet as the train moves closer).

Scenario #5 - Train calling foreman 

When track repairs must be made, the crew's foreman sets flags outside of the work limits. Red flags are put lineside no less than 200 yards from the work area, and yellow/over/red flags no less than 3000 yards from the work area. 

Construction foremen radios are usually always on channel one. Trains, on approach to a construction zone (indicated by red flags), communicate with the foreman via radio. The name of the foreman and nature of work are designated in the DOB (Daily Operating Bulletin - a printed list of all of the unusual conditions happening on the particular subdivision, such as slow orders, construction (with mileage and foreman's name), unusual track conditions, etc.). 

633: "633 west calling foreman George Cameron, over"
Foreman: "Foreman George Cameron go ahead 633, over"
633: "633 west approaching red flag mile 14.2 Halton subdivision on the north N-O-R-T-H track, leading with engine #5601. what are your instructions, over"
Foreman: "633 west approach north track no restrictions, with bell and whistle around men and machinery, over"

The train then repeats and confirms the information before passing through the construction zone.

These types of communications are VERY handy to the railfan, as it not only tells you that a train is on its way, it also gives you the train number and the type of power leading the train (using the engine number).


Scenario #6 - TOPs (Track Occupancy Permits)

In order to travel to and from worksite, or to perform certain kinds of maintenance on the line, it is necessary for work crews to travel on the rails. In order to protect the crew's puny hi-railer from a chance head-on encounter with a hurting freight, the RTC issues Track Occupancy Permits to the work crews. This allows the crew to travel between two designated points (usually signal numbers or station names), and be protected from surprise visits.

RTC: "TOP number 653 six-five-three foreman George Cameron permission  to occupy the south S-O-U-T-H track between signal number 142 one-four two and signal number 185 one-eight-five of the Halton sub. Please call RTC before 1400 hours".

This is handy for the railfan - it allows one to get a feel for the day's traffic patterns and know of any construction on the line (red flags are a bonanza for the railfan who likes a little notice before a train comes).

As well, "Call RTC before xx hours" is handy in determining when the next train is due. If the time posted is two hours from now, you can expect a train in two hours.