Cordless phone interference has always been a problem–phones interfering with other things, and other things interfering with them.
That was the draw of 900 MHz phones. There wasn’t anything else running on that frequency at the time, so there was little to no interference. But 900 MHz didn’t sound hi-tech in the age of gigahertz computers. So in the early 2000s, 900 MHz gave way to 2.4 GHz and 5 GHz phones. That brought back the problem, because there’s so much other stuff operating at those frequencies these days, like wireless computer networks. But there is a solution that doesn’t involve digging up a 20-year-old 900 MHz phone and trying to find a battery that works in it.