Vodafone UK lost 27,000 customers during the last quarter with its domestic customer base now standing at 18,510,000.
In the UK quarterly voice revenue dropped from £860m last year to £822m this year, but messaging revenue rose from £208m to £236m and data revenue went up from £84m to £110m.
Across the whole group, the Q1 results were in line with analysts’ expectations, but Voda bosses warned full year revenue will be at the bottom end of their outlook range of between £39.8bn and £40.7bn.
Shares in Vodafone dropped 12% after the cautious outlook wiping £9bn off the value of the company.
The results showed second quarter sales were up 19.1% to £9.8bn with data revenue up 50% to £664m.
Voda chiefs said they are expecting the group to finish the year with an operating profit of between £11bn and £11.5bn.
The report is CEO Arun Sarin's last as he will step down on July 29 after five years as the head of Vodafone, to be replaced by his deputy Vittorio Colao.
Sarin said: ‘Notwithstanding this more challenging operating environment, we continue to benefit from a diversity of assets and services, with strong revenue growth in EMAPA and another good quarter of data revenue growth.
‘Whilst we expect revenue around the bottom of the outlook range, our continued focus on cost reduction enables us to reiterate our operating profit and cash flow guidance for the year.’