AVOlio wroteWhere can I find vdsl splitters.
As far as i know, the modem needs to be directly connected to the phone line (which is what the 'modem' port does in the adsl/vdsl filter). The filter is only actually "filtering" the phone line, so shouldn't really matter.
Try a different filter, it may be broken or low quality. For example, i have one cheap Chinese filter that for whatever reason gives me 20% lower speed. Using the filter that came with the modem gave me near full speed.
Try your filter first connect the modem to it but not the phone. If the speed goes down, it is definitely a faulty filter and you change it.
If the speed still the same as without the filter, then it could be your phone. Try another phone (borrow one) and try it.