For example - a call in the evening to another landline for one hour - 5.5p (ok, 10.5p if you include the 5p connection charge).
The equivalent call from my mobile to another network - £24 (£0.40 x 60)

In fact, make that for all calls to and from mobiles. Calls to and from mobiles are a complete rip-off unless you pay for extra tariffs.
This is the only phone I've ever had, and the only network I've ever been on, so I'll vote O2.