Contract lengths are a trade-off between cost and commitment. Commit to 18 or 24 months and your reward is a lower rental and a no-cost or lower cost handset. Go for greater flexibility with a six or 12-month deal and you pay more. For example, for an iPhone 3G S 16GB handset with 600 mins/500 texts, on 24 months you’d currently pay £87 compared with £185 on 18 months.