Check first if the S25 register on your modem determines DTR drop time. Some modems are not set to the proper factory default. More info . More info . It is even more critical with poor quality phone lines or when calling from outside Bangkok.
Rain often degrades phone connections.
Connect speeds of 28.8Kbps are possible even on noisy lines, so a V.34 modem can cut down your online time a lot and adapt to line conditions. All modems are not the same. Don't buy RPI modems or modems which only work with Windows. The USR Courier V.Anything is reputedly the best on the market. Other good brands are Microcom and Motorola.