International operators should allow no tolerance in their assessment of the chargeable duration of calls. Metering devices controlled by operators should be rapid in action and have the utmost accuracy. In the automatic service (and, in the case of station calls, in the manual and semiautomatic service) the chargeable duration should begin from the receipt of the answer signai from the called station (see the definitions in Recommendation E.100 [l]), since the existence of an unchargeable call period, however short, might lead to misuse of the service for the transmission of short messages without payment.