The wireless headphones that utilize a digital transmission signal operate in the unlicensed 2.4 Ghz range as do many other wireless devices (like WiFi routers, wireless printers, etc) and the headphones have a specific frequency range on which they can operate. The headphones scan the local wireless signal environment and then automatically select an unused frequency within their designated range. The headphones are designed to work in conjunction with other wireless devices without the different devices interfering with each other.
Typically the bandwidth of the WiFi router is greater than that of the headphones and it actually encompasses the bandwidth that the headphones want to use. Most of the time the two signals are away from each other and there is no issue. Occasionally if the WiFi router has established it's signal in the bandwidth that the headphones want to use then the two signals butt up against each other this can cause issues. Sometimes it is possible to change the WiFi router frequency manually by using a switch on the router itself.
In situations where the WiFi router cannot be changed manually the simple solution is to turn off the headphones and the WiFi router. Start the headphones and let them run for 15 minutes to establish their signal and then turn on the WiFi router. The WiFi router will "see" the signal of the headphones and establish it's own signal in a different part of the bandwidth and the two devices will work without interfering with each other. NOTE: Once this process has been done the signals are established and will remain established even with a device is turned off.
If the above solution does not resolve the issue it may be a situation where the wireless environment is too crowded and the multiple wireless signals keep butting up against each other and causing issues. To resolve this some wireless signals would need to be removed from the environment (ie: turned off). For example if a wireless printer was turned on it is creating wireless signal that would be part of the wireless signal congestion. If the printer is not used all the time then it could be turned off to reduce the congestion and allow the other wireless devices (like the headphones) to work.