Dial tone in any swithing system has to do with the arquitechture of the system. Upon de system or brand you have will work the dial tone distribution, for example some system internally in the line board has the dial tone distribution, in the system I know this value is 20 but all the module is 128 lines, therefore it is that 20 out of 128 can have dial tone at the same time other will have silent, using this for the total line cards or module you can calculte teh mount of user. My recommendation is ask for this capacity to your provider or vendor at least per line module then calculate the total.
As per Luigimex the dial tone caluculation is corect I think, but it is for particular model. If it is for public switch, now a days every connected subs will get a dail tone which is virtual dial tone. They will get the congesstion if any so. All these things come under the Time slots[TS] technology. So Matt pls let me know what is your conmfiguration for the TS in your system and also what is the max. subs you are going to support through your switch. I will try to caluculate in gerneral based on your fed back…