The European Commission says most mobile phone operators are complying with the regulation to cut mobile roaming charges by up to 70% in the EU.
Mobile operators say competition, not regulation, is the key
About 50% of consumers in Europe are benefiting from substantially reduced mobile phone tariffs, EU Telecom Commissioner Viviane Reding said.
The deadline for introducing the new "eurotariff" was 30 July.
A cap of 49 euro cents ($0.67) a minute was imposed for making calls abroad. The cap for receiving them is 24 cents.
The caps followed a lengthy investigation into the market by the European Commission. It calculated that mobile phone firms made about 8.5bn euros ($11bn; £5.7bn) profit a year from roaming fees.
Some operators were offering the reduced eurotariff already at the beginning of July, the Commission says.
The cheapest tariff so far identified by the Commission is 31.5 cents a minute for calling and 12.6 cents for receiving a mobile call abroad.
On the negative side, the Commission noted that an operator in Cyprus, Cytamobile, would be offering a eurotariff only from 30 August.
The Commission has threatened sanctions for firms violating the regulation.
The Commission said a number of firms did not respond to its request for information, including all those in Latvia and the British operators O2, T-Mobile and Virgin Mobile.