A wireless router and wireless network adapters can have interference issues with cordless phones. It depends on the network adapter frequency and the cordless phone's frequency. Most problems will occur with a phone that uses 2.4GHz. This frequency will interfere with 802.11b/g networks and single-band 802.11n wireless signals.
A DECT cordless phone operates at a different frequency from the 802.11g and 802.11n single-band and will not interfere with a wi-fi network with those modes.
However, some routers and wireless adapters can use the 5GHz range (802.11n and 802.11ac, among them). These can have a problem with cordless phones that use 5.8GHz. However, there is a simple solution to avoid much of the interference at 5GHz. First determine the frequency range of your phone by getting the FCC ID from the base of the phone. Then use Google or another search engine to find "FCC ID Search Form". It has been at http://transition.fcc.gov/oet/ea/fccid/
in the past. The first three characters are the Grantee ID for the search form. The rest is the product ID. Look for the SAR report for the phone and then (probably on the second page) find the TX frequency range and the Max RF output power tested. Now go to the wireless router's setting page and make sure that it is using channels that do not use the same frequency and power range. See http://bradsfavorite.com/avoid-interference-5-ghz-wifi-5-8-ghz-cordless-phones/
below the 5GHz Wifi Frequencies for a table with the channel and frequency ranges for routers.
Once everything is clear, you can visit ScanmySpeed.com
and have an internet speed test to ensure that there is no disturbance in your connection.