I'm really hoping someone has an answer to this as its got me confused!
I'm on O2 and have a K800i, my wife's on Orange and has a W800i. She called me yesterday using her speed dial and ended up hearing a strange glaswegen girl talking rather than me as she expected. I had a missed call on my phone as i hadnt even answered. This happend a a few other times, some with people answering other will a strange error like tone.
It turns out that i was using the browser at the time (opera mini). So i tested it at home calling my phone from hers while i connected to bbc.co.uk. Sometimes it went to my voicemail, sometimes just silence and about every sixth call or so it went to random people. Each time that happened i spoke to them to see who they were, but they clearly couldnt hear me as they kept saying "hello", "who's there" etc.
This has got me baffled.. Who do i go to, O2 or Orange?? I'm betting they'll each blame each other. Thankfully it happens so often i can demo it.