Bandwidth loss when registered to VoIP-SIP server

Hello Everybody!
I bought my SpeedTouch 716g in the UK and using it in Istanbul/Turkey. When the ST 716g is registered to a SIP VOIP provider-server (eg. voiptalk.org, voipfone.co.uk), it allocates 1/3 of my bandwidth, even when no phone talks are being made!!!. My downloads fall from 53KB/s to 33KB/s (have a 512/128 ADSL). Unregistering from the SIP provider brings download speeds to the normal rates. You can be sure that no other application uses the bandwidth during this time. Everything else works fine with the router and the phone quality is ok. My question: Is this bandwidth loss normal even just in registered mode and no voice calls are active?
I have the latest firmware R1.0d installed and tried all the possible (Qos) settings, with no success.
Thanks very much for your help.

Some details:
ADSL 512/128 – 8/35 PPPoA-VC MUX
UBR with PCR
Alcatel DSLAM
QoS enabled
QoS: Default settings, no IP QoS setting added