Agreed, though I'd interpret it slightly different. Like if I said 100 cars per hour and each car needs 30 seconds of spacing per car the units don't cancel out

Agreed, though I'd interpret it slightly different. Like if I said 100 cars per hour and each car needs 30 seconds of spacing per car the units don't cancel out because they are physically taking up space for the "30 seconds of spacing per car." I'd say the same applies in this case. It'd be more "calls/sec" times "active phone * sec/calls". Though I guess for "call" in your case or "active phone" the industry has chosen 'Erlang' a bit recursively.