Nigeria's telecoms regulator has ordered MTN to increase its tariffs after it found that the cost difference between on-net calls and calls to other networks was too great.

The regulator said that it was concerned that the mobile voice market is not effectively competitive and is still highly concentrated with MTN having a 44% market share of subscribers within this market.

In a ruling it said that the low cost of on-net calls was "indicative of the likely establishment of a calling club for MTN subscribers".

As a consequence, the regulator ordered that the tariff for on-net and off-net will have to be the same, and subject to periodic review. The change is to come into effect immediately.

The review also found that MTN and Glo jointly control about 62% of the public terrestrial transmission infrastructure which is a bottleneck resource in the provision of voice and data services. The report expressed concerns that Operators playing in the wholesale and retail sub-segments of these markets have the leverage to "squeeze" the margins of their competitors who are also their customers.

As such, the two companies are also be subject to more stringent regulations covering their operations.