One of the companies at the center of the investigation used to be a state monopoly.
Mexico’s telecoms regulator has opened an anti-trust probe into the domestic market for public telephones, the government said in its official gazette on Tuesday.
The investigation will look at possible anti-competitive behavior in the “market for production, distribution, and commercialization of public telephone service for sale to final users via telephonic apparatuses for public use,” it said.
Telmex, a unit of tycoon Carlos Slim’s America Movil, dominates the fixed line phone business in Mexico.
Before Slim acquired Telmex at the start of the 1990s, it was a state monopoly. It has public phones throughout Mexico, where it competes with smaller firms including Logitel and Alta.
As is customary, the government did not say which company the Federal Telecommunications Institute (IFT) was targeting in the probe, which was sparked by a complaint.
Local media reported in July that another company, Conatel, had made an anti-trust complaint against Telmex over the public telephones market. IFT documents published online in June showed Conatel had complained it could not compete with Telmex.
Get Data Sheet, Fortune’s technology newsletter.
Consulted on the matter, a Telmex spokesman said it was too early to say whether the company was the target of the probe.
Slim, Mexico’s richest man, has dominated the local phone business for years, and the government passed a major telecoms reform in 2013 to loosen his hold on the telecoms market.
The public phones probe will last at least 30 working days and not exceed 120 working days, though the period of investigation can be extended if necessary, the government said.