It is a dimensioning calculation similar to estimating circuit quantities. Quantify the average holding time of the tone detector, quantify the BHCA to the detectors, caculate the ‘Erlangs’ of traffic to the tone detectors, assign a high grade of service ( eg .001) and use Erlang C.
Erlang C because it is a queueing situation, the call MUST wait til a tone receiver is available.
After you have calculated the quantity required, add some more !!
You must be very sure of the devices which will access tone receivers, The mey be used for calls other than telephones originating calls. I had an instance (some years ago) whereby tone receivers were used for each Direct-in-dial call, When busy hour traffic occurred there were high failure rates as the tone receivers had only been dimensioned for outgoing/originating traffic.