Try other cordless phones. If any of the other cordless sets work, you've found the problem.
The interesting part is how makers will respond to you about this.
I have an Orange Livebox connected wirelessly to my laptop. Everything's been working fine until the last few weeks. Whenever I receive or make a call using my iDECT s2i cordless telephone the internet drops and it takes ages to reconnect. I've checked all the connections and every phone socket has an ADSL filter. I understand that some wireless devices can affect a router and that these are usually 2.4ghz devices but I have no idea if my cordless telephones are 2.4ghz or 5.8ghz or indeed if this is the source of the problem at all.