An inquiry has been launched by the European Union into whether blocks on VoIP service by Europe’s mobile phone operators are in breach of competition laws.

The EU’s antitrust authority has written to every mobile phone operators how they deal with internet based voice calls.

The deadline for responding to the letter was Wednesday.

Several phone operators have clauses in their contracts which either block VoIP services over their 3G networks, or exclude VoIP traffic from monthly data bundles included with the tariffs.

Some operators have also tried to block the VoIP over Wi-Fi capabilities in some handsets.

Last year, VoIP provider, TruePhone threatened to sue Vodafone after the operator was alleged to be blocking calls over its VoIP network.

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