I suggest trying an ADSL Nation filter, see how that holds up. There is a faceplate type and a dangly type (the XF-1E) and I believe they are of similar design. If memory serves, these are one of only two designs that have transistors in them, the idea of this complexity being that it copes well with the differences caused by varying load between the on-hook and off-hook situations where there could be various types of phone and varying numbers of phones and so on. This means that the load presented to the filter is unknown.
Apologies if this is a bit vague as it's a long time since I read up on this. I read a chapter on microfilter design in a DSL text book. The other transistorised filter iirc is a French design, can't remember who, and I failed to get hold of one to try.