None use WiFi , your cordless ‘landline’ phones are just the same as any other domestic cordless phones that have been around for decades, in that they are DECT , not WiFi , your WiFi isn’t used for telephony unless you have WiFi calling on a mobile phone ( not a cordless phone) …the adapters are DECT adapters, they just allow a corded phone to connect to the DECT basestation within the router .
Your calls that don’t ‘connect’ , is it one number you call that’s intermittently failing to connect or various random numbers , and is the number (or numbers ) other ‘BT’ customers or is it (or they) on other providers networks ( Sky , Talk Talk , Vodafone, O2 etc ) .
If it’s only one BT number (you ) to another BT number then that call is contained entirely within BT area of responsibility , but it may not be that easy to tell as people can move numbers around providers so you may not know who is responsible for terminating the call , if it’s definitely to another providers network, then if the issue is within that other network, clearly BT can’t ‘fix’ other network operators equipment, unfortunately with intermittent issues like this it’s very difficult to rectify as no one has control over every area .
