As of a few months ago, I started having problems with my ADSL connection. The DSL indicator on my Router just kept on flashing (suggesting that it could not establish a connection with the ISP). (Interestingly, this phenomenon occurs only during the day. After about 17:00, everything works just fine.)

A few week ago, (after repeated attempts) I actually got a visit from a TOT technician (Yeah, I know. Shocking). As best I understood him, he said I should purchase some “box” to sit between the phone line and the Router, to somewhat boost or filter the signal.

I then went to Central, but no one seems to have heard of this magic “box”. Can anyone please enlighten me?

The thing you want is called a 'filter' or 'splitter'. You should be able to get them from any shop selling routers etc at Fortune Plaza or one of the other IT malls. They look pretty similar to one of those little plastic boxes you use for plugging in an extra phone extension. They separate the phone/ADSL signal to stop them intefering with each other.

Cost somewhere in the vicinity of 200 baht. As an interim solution, pull your phone plug out of the wall

Did you check the box your ADSL modem came with. Usually is supplied with the modem. Has 3 RJ-11 (modular phone) jacks on it, one to the Line, one to the modem and one to the phone. Used to split/filter the low frequency audio from the high frequency modem carrier frequency. Without it you can get interference between the two spectrums. Another possibility is your line QOS (quality of service) is poor due to bad wiring. But you certainly should have the splitter on the line.

Make sure to get a good quality splitter, such as one from 2Wire: