We present a calibrated model of the UK mobile telephony market with four mobile networks; calls to and from the …xed network; network-based price discrimination; and call externalities. Our results show that reducing mobile termination rates broadly in line with the recent European Commission Recommendation to either "pure long-run incremental cost"; reciprocal termination charges with …xed networks; or "Bill & Keep" (i.e. zero termination rates), increases social welfare, consumer surplus and networks' pro…ts. Depending on the strength of call externalities, social welfare may increase by as much as £ 990 million to £ 4.5 billion per year, with Bill & Keep leading to the highest increase in welfare. We also apply the model to estimate the welfare e¤ects of the 2010 merger between Orange and T-Mobile under di¤erent scenarios concerning MTRs, and predict that consumer surplus decreases strongly.