My problems with splitters/microfilters seem to have been shared by many others. I initially bought a set of cheap ones (£7 each) and these caused problems with sound quality on the telephone. Other members of my family found it really bad using the 'phone (although I myself didn't particularly notice the difference, but my hearing is not as sharp as theirs!).
I finally replaced the splitters with some more sophisticated ones, albeit twice the price, and they seem to have made all the difference. They sell at £14.95 and can be bought from <A HREF="www.lightwave.ltd.uk" TARGET="_new">www.lightwave.ltd.uk</A>. One unit out of four was actually faulty, but hopefully that was a one-off.
Perhaps the lesson to learn is to not cut costs by buying cheap splitters.