the number thing is weird
When I switched to Vodafone I forgot to tick the "I want to keep my number box"; I realised after ordering but before it was set up. When I called to say "hang on a sec I don't don't a new number" they told me they couldn't let me keep the old one anymore as they weren't allowed to transfer it but they could cancel the whole process. I figured sod it that is a few cold callers who won't have my number...
The service itself - for months/years they refused to tell anyone how to use their own router (they had family based ussge control focus to start), the supplied router is pretty poor, and had many weird quirks including at one point not letting the LAN devices talk to the WiFi devices.
I don't get sync speeds anything like BT/Vodafones tool think I should get 41M instead of "61 minimum" but I have no idea if that is the router or the fact my house still has a twin solidcore drop not the modern multicore cable. Like any other ISP the DNS is pants but otherwise uptime has been perfectly adequate and it is cheap as chips and has got cheaper. For gaming I get pings under 40 for a decent number of servers and Netflix/Prime/iPlayer work as they should, what more do you need?
From the (terrible) forums I get the impression the support if you need it is shockingly bad and from experience if you use the online support they will take you through your security questions check your account and then tell you theres nothing they can do you have to call the sales line (all I wanted to do was renew).
Back when self install was new I used Zen (and 8IPs because old routers couldn't handle the NAT table for hefty piracy).. it was great but if you don't need the support, why pay for it