Hi.
If 1 mA; or 5 mA or more are taken from the on-hook telephone line, nothing will happen if the dial tone is not present and the incoming call will ring with no problem.
Only when the current taken from the telephone line exceeds the treshold to engage the central office relays, usually around 18 mA, then the dial tone and the busy tone are enabled and the ringen disabled by the relay contacts, preventing an incoming call from passing trough.
Modern central offices may not use true relays in their linecards, but the functionality persists the same, and some automated test may flag a current being taken as a failure, a leak, or an intentional load.
Miguel