The only things which will pull down speeds are those which are using WAN/Internet connectivity.
VOIP will normally have keep alives - measurably in ten of bits per second (average). There will be an occasional - "I am here" type message to maintain registration.
A WiFi network will not - unless you are providing a BT WiFi hotspot when there could be occasional traffic or a roaming user.
Speedtest from http://www.thinkbroadband.com/speedtest.html
Modem stats: Sync Speed, Max Attainable Speed, attenuations and SNR.
M H C
taurus excreta cerebrum vincit