Appending to the advice Beoaus has already provided you, BeoLink Wireless 1 is based upon the WLAN (Wireless local-area network) technology (but not using the IEEE standard for WLAN). This technology is using the 2.4 GHz band which is also used by many other wireless devices, for example WiFi, wireless LAN transmission, cordless telephones (DECT) and interestingly microwave ovens or at least cause problems at that frequency.
As Beoaus says it is important to have good channel seperation with your wifi, but even with this channel seperation if the units are physically near each other (ie the BeoCom 6000 PSTN and Wifi) you may still have problems.
Having said all that, how sure are you that these products are the cause of your problems. Have you switched off the wifi and DECT phone to see if you still have the problem ?