I don't have line rental with Plus.Net but this glitch stops me from viewing the page (most of the time).
I think I used Firefox on one Android mobile, Dolphin on the other. I wondered whether it was to do with my IP and being an existing broadband customer, but oddly, the phone using unlimited data on Three network did NOT show the page, while the phone using Wi-Fi and my Plus.Net ADSL was redirected.
In case it was to do with cookies, I tried both Chrome and Opera on my other iMac, and neither showed the /line-rental-saver/ page. I hardly use Opera (and Chrome is only on 2 of my 5 iMacs, because they're all old but 2 use Intel CPU and can run Chrome, the rest are on older versions of OSX and the PowerPC G5 CPU).
Seems odd that in some situations it displays but in others it redirects. I think if there's an exclusion (if it also depends on your phone calls package) they should put that as a footnote, not just hide the page!
If you run a business, have a second ISP and backup web hosting...