Phone affecting speed

[#1]

Quick history.  Having intermittent noise problems with phone.  Went through a process of trying different filters, a wired phone and so on, trying to isolate the cause.  Eventually a BT engineer was despatched and he decided to change the socket, upgrading me to a modern socket.  To my relief this cured the noise problem.
However while doing the testing I notice the connection speed the modem sets is dependent on the filter I use and the phone (This is backed up by subsequent speed test - speed range 7.5 to 9.5 Mbps).  The filter is easy to sort, I use the best one (I had 3 to try).  The phone is not so easy to sort as I get the best performance with a wired one but I obviously prefer suing my wireless one (as there are several units around the house).
Any one else see this?
Any one else recommend a phone that does not affect the connection speed?
S

[#2]

Historically DECT phones have caused more issues with broadband than other types.
I've not heard of any issues/seen any posts about that in recent years though.

[#3]

Thanks for the info. Adam,
Now I look at it again I realise that the big improvement I made, aside from the new main socket, was using the best filter I had.  The difference between the phones is 8.5 Mbps to 9.5 Mbps and I can live with that.  Rather than change the phone, or lose some of the flexibility.  On top of which choosing a phone is a bit of a nightmare.
Thanks again S

[#4]

You might try using two filters with the cordless phone. Until I did that I was getting disconnections every couple of days, but once I plugged in the second one my line would stay up for months at a time. (I suppose it's possible that the first one was bad and just swapping them would have had the same effect, but why mess with success?)