THE volume of international voice calls made using Hongkong Telecom's card-based services grew rapidly last year, according to Mr Peter Wong, manager with the company's international telephone. ''Our major service introduction last year was Auto Hongkong Direct, which allows holders of our international telephone card, Calling Card, to dial directly back to Hongkong from any telephone in countries where the service is available,'' he said. ''Auto Hongkong Direct was introduced at the beginning of May from six European countries, and was subsequently brought in from Canada and the United States. In October, Auto Hongkong Direct service expanded to include 19 major cities in China and, in December, it became available from Britain and Singapore. ''By the end of December, the volume had increased to more than 28,000 calls a month from around 300 in the first month of the service,'' said Mr Wong. The corresponding outbound Calling Card service is IDD 200, introduced in May 1991 for calls to any IDD destination from any telephone in the territory. IDD 200 has also experienced rapid growth. For the final six months of last year, about one million IDD 200 calls were made, compared with more than 500,000 for the previous six months. ''The most IDD 200 calls are made to cities in China,'' said Mr Wong. ''China accounted for 50 per cent of IDD 200 calls following the introduction of another 358 IDD destinations in November and December.'' It is now possible to make calls to more than 1,000 destinations in China. More than half a million people are holders of the Hongkong Telecom Calling Card, which has one of the highest per capita penetration rates of any international telephone card in the world.