I am not an EMI (electro-magnetic interference) expert, but I believe many mis-conceptions regarding possible 900Mhz interference with ADSL are completely unfounded. ADSL operation in the 50Khz - 1.1Mhz frequency range...no where near 900Mhz. Therefore, I am pretty certain that any interference is NOT from the 900Mhz signal itself.
Recently I've heard a plausible explaination. There are many poor-quality portable phones on the market where the wired portion of the phone (vs. the wireless) has very high resistance (for those familar with REN, a high ringer equivalency number). Such phones can disrupt ADSL signals even when used in conjunction with a micro-filter. However, this would only be the case when the phone is used on the same phone line as ADSL AND I would think would only be an issue when distributed micro-filters where used vs. a centralize splitter.
Another explaination is as follows:
The 900Mhz signal generated by cordless phones is generated using a "core" frequency that is much lower. The process of multiplying this "core" frequency up to 900Mhz also tends to generate significant noise on frequency bands between the "core" frequency and 900Mhz. Depending on this specific "core" frequency, ADSL frequencies can be impacted.
I personally use a Lucent 900Mhz DSS and a Ericsson 2.4Ghz DSS multi-handset system and have ZERO interference problems. Of course I also have a dedicated copper pair all the way from my house to my mini-RAM, so I don't even have a splitter or micro-filter of any kind--no possibility of phone equipment interference.