I am not certain since I am not there and cannot check with a Spectrum analyzer, but those Wireless Headphones use the 900 Mhz Spectrum and many devices use that frequency especially Regular Wireless phones and also some low end routers. You might be getting some other device overriding the frequency that those Headphones use.
I personally use Bluetooth headphones since they are Adaptive Frequency-Hopping (AFH) and they use a different frequency than Household phones and routers. Plus they are cheap or expensive and they are new school not like your old school headphones which yours are since they are not Adaptive Frequency-Hopping (AFH) like the Bluetooth are.