Re: Reader Kantanit's problems of random disconnects with his iMac (Post Database, January 26, 2000), there are no surefire answers but several things that can be tried. I assume the iMac is new and shouldn't have any hardware troubles at this early stage. (Although this isn't an impossibility. I have a friend in the U.S. who had his iMac die soon after purchase.)

1. Definitely use the Telecom Asia direct line for now. I use a TA line at home and it works okay, although others have noted problems with their TA line (mainly having to do with getting sub-par speeds). On the other hand, using an extension line shouldn't necessarily cause problems. At my former place of employment they use extensions and it works fine (although there's a speed drop having to go through an inordinate # of "exchanges").
2. Could reader Kantanit try surfing during off-hours, like very early in the morning (say starting at 6 am). Try it for a 1/2-1 hour and see how it works. Any disconnections?
3. If the problems persist, can he try limiting the modem speed to 33.6kbps. I presume the iMac came with a 56kbps modem. Unfortunately, I don't know how to do this on a Mac/iMac. Many "home" or "value" computers tend to come with less than "premium" modems to save costs and this may or may not be the case with the iMac. Some modems don't do 56kbps well, but all modems should do 33.6kbps alright.
4. There are at least 2 stores at Panthip Plaza that sell Mac's - including iMacs. They're on the upper floors, one next to the escalator and the other one near the center "opening". It might be worth dropping in and asking them about this problem given their specific Mac expertise. It provides a second opinion. I assume dialing out of Panthip Plaza given it's age and other messiness would be fairly "challenging" so they may have experience with modeming in less then ideal situations.
5. While he's there he might ask about what other kinds of modems can be installed in the iMac. For example, I don't know if the iMac can accept regular external serial modems. Or perhaps the iMac only takes USB modems? Information worth knowing since he's already dropping in on a vendor. I don't recommend that he buy a replacement yet, not at this stage.
6. Could reader Kantanit try his iMac somewhere else? Say, at his office, at a friend's/relative's home? Given it's all-in-one construction, it shouldn't be too difficult to move around.
7. One final thing, call waiting may be turned on on his TA line or even his old line. This needs to be disabled, otherwise a phone call into the line may cause the connection to drop. Internet Thailand explains how to do this at: https://www.inet.co.th/support/cnct-mac.htm.

The objective of all this is to find a situation where the iMac works in a stable fashion and to see what's unique about that situation.