CALL-BACK services can slash 50 per cent off the cost of international calls but technical problems and inconvenience can disappoint users. Picking the right service can help avoid many problems but the variety and rapid increase in numbers make it difficult to make a choice. Call-back services exploit differentials in the international call rates charged in different countries. A consumer dials a number in the US, for instance, where the call-back computer is located. The computer identifies the caller without answering the phone, thereby eliminating any immediate charge to the caller. The caller is called back when the appropriate connection is made, and the outgoing call from Hong Kong becomes an incoming call from the US. The customer later pays the call-back operator the US rate, which is almost invariably a great deal cheaper than calls made through Hongkong Telecom. Most call-back operators are based in the US or Canada, where there is excess capacity in international telephone lines and the charges for calls are relatively cheap. There are currently about 30 call-back operators providing services in Hong Kong, varying from large operations such as City Telecom, which has more than 180,000 users, to backyard operations run by phone boffins. The new fixed-line telephone operators in the territory - Hutchison Communications, New World Telephone and New T&T - offer services akin to call-back as a way of getting around Telecom's supposed monopoly on international call services. However, these differ from straight call-back as they are reselling access to Telecom's own international gateway rather than those in other countries. The differences in charges between Telecom's IDD rates and those of call-back operators can be over 50 per cent on selected routes. World-Interlink, for example, charges $3.60 per minute for calls to the United States, while Telecom's IDD rate is $6.80. However, there are hidden costs and charges associated with the various services. Consumers have to watch out for poor service and company charges, such as minimum rates, sign-up or subscription costs, or charges for calls unanswered or engaged. There are also technical glitches. One user complained he was occasionally woken up at night by the call-back company confusing his number with that of a caller. When comparing call-back services, a consumer should ask for information about up-front fees, monthly charges and minimum fees. It is also difficult to keep track of the call charges. They change frequently as competition leads to constant undercutting. City Telecom, for instance, has just announced a VIP rate offering discounts for bills over $400 per month. The operators surveyed had a range of different charge structures. AIC Telecom, City Telecom and ElephantTalk said they charged no extra fees. But ElephantTalk does have a minimum call length of 30 seconds. KallBack charges US$100 and a monthly fee of US$10. PhoneSaver said it had no additional charges but proved extremely difficult to contact. Union Telecom requires a deposit of HK$500 before making a connection. An obstacle many see to joining a call-back service is that most require you to deliver your credit card number in advance. There is also the problem of delays. Waiting time for the return calls varies, though this is being reduced with auto-dialers. Another problem is the uncertain regulatory environment for call-back services. The services have already been outlawed in a number of countries.