This is not like licensing an out of the box solution. So, it should not be one fee and done (very doubtful that would ever be the case). One fee and done would leave TELMEX in a very tough situation as technology evolves.
If there is a major license, then part of that license will include continuing support fees and continuing maintenance (development of enhancements and evolution of technology for changes in involved third-party interfaces).
Typical major licenses like this would involve ballpark of 17% in annual "maintenance fees" plus an hourly rate structure as a wrap-rate for continued support. First few years the continued support would be very significant. Remaining years the profit model on maintenance should be manageable if the product original structure was designed for modular maintenance (surely it has been).