The way we have done it is mainly with 1 person doing all the calls. The calls just keep adding up as you, and if stuck with passenger calls come thru then of course they get priority. It will come a time when 2 are required.

I would advocate that the best way to do a 2 person roster is to do a next cab of the rank style ie pick a name as the first on the night( at the start of the o/t), then flip it for the next night and so on. If it is on paper, then no-one can get upset about who takes the call. Of course you could also have an agreement with the mechanics if call come up, which are close to a mechanic to flip the order of the calls, especially if they are stuck with passenger calls.