Switched mobile phone service to O2 this weekend. The only major UK mobile telco I haven't had an opportunity to hate yet.
I'm doing well at it already -- their network doesn't even handle delivery status reports for text messages. I send a message with a status report requested, watch it get delivered to the destination.... and watch it still reported as 'pending' on the phone I sent it from. Every other network can manage delivery reports -- but O2 don't seem to be able to do it.
They can't get voicemail notification right either -- they send you a text message telling you that you have voicemail, instead of just turning on the voicemail icon on the handset.
99% of times I get notified of a voicemail, it's either because I'm busy -- in which case I've just rejected the call and I don't want to listen to the message now, or because I've been out of network coverage -- in which case I don't want to listen to the message now because I'm probably driving, or I've only just got enough coverage for the _text_ to work, not for voice yet. This means I look at their silly text message, don't bother to check the message immediately, and promptly forget that it exists, because it's no longer a new message.
The GSM standard defines a method for turning on a 'voicemail' icon on the handset, which when used properly will stay on for as long as there is voicemail waiting on the system -- it's far more useful. But O2 don't seem to be able to manage that either.
Fucking Useless Telco.