Happy with your handset? Choose Simplicity. No lengthy contract. Just 30 days' notice to cancel. Commit to 12 months and O2 will give you 33% more minutes and messages.
You will get 600 UK minutes and 1200 UK texts each month, free voicemail and itemised online billing. At only £19.58, it is a very good deal that's competitive with T-Mobile's own Solo offer.
Stick with Simplicity if (a) you can afford a 12 month contract, which will give you 800 minutes and 1600 texts instead (b) if you want to have broadband at the same time, O2's award winning 8Mbps Broadband will cost you a mere £7.50 per month.