They're cordless phones, not cellphones. I haven't used the 5.8 GHz phones, so I can't opine on them. The 2.4 GHz phones seem to me to be less susceptible to interference than the 900 MHz phones, but that may be a matter of how many users there are in a given band. I have been using 2.4 GHz phones in my home for many years, coexisting without incident with wireless networks using the same band (initially HomeRF, now 802.11B/G). Currently I have three Wi-Fi access points running, along with 8 AT&T/Vtech 2-line spread spectrum handsets and three Wi-Fi computers (more when my son's friends come over with their laptops to play games). Also a microwave oven. There is a small amount of occasional interference on the phones, mostly location-based (i.e., in a bad reception area), and no wireless issues to mention with regard to Wi-Fi.
I had bad luck with Siemens Gigasets (lots of interference and very very poor battery life), better luck with AT&T/Vtech. I had a wonderful Uniden many years ago, but it was discontinued.
Michael D. Sullivan Bethesda, MD (USA) (Replace "example.invalid" with "com" in my address.)