In the UK, T-Mobile and Orange To Merge
EthanV2 sends in BBC coverage of the merger plans of Orange and T-Mobile in the UK. "T-Mobile and Orange plan to merge their UK businesses, creating a mobile phone giant with 28.4 million customers. If completed, a deal between Deutsche Telekom's T-Mobile and Orange owner France Telecom would see a firm with sales of €9.4 B (£7.0 B, $13.4 B). It would be the UK's largest provider, overtaking Telefonica's O2, with about 37% of the mobile market. ... However, it is likely that competition authorities in the UK and EU will probe the deal."
Both the EU and UK (Competition Commission and/or Ofcom) regulators will be paying a great deal of attention to the merger, given the high barriers of entry to the market place (i.e. it's not really possible) and the low number of competitors.
I'm pretty sure that anything over 25% would legally be a monopoly in the UK (I know technically it's not a monopoly, but in a rare case of foresight the monopoplies and mergers laws took into account that 25%+ is enough to distort a market).
If you read the summary, it is about 37% of the market. There are more cellphones than people in the UK. Pretty much everyone has a cellphone, and a lot of people have more than one - eg work cellphone and personal cellphone.