The discounts are laid down in Article 38 of the Annex 1of the Act of 13 June 2005 on electronic communications.
Several hypotheses are summarised in the table below:
€ 11.5 (if Internet)
or € 3.1
* NB: if you pay the subscription fee and the communications to two different operators, only the operator providing the communications grants a maximum discount of € 11.5 on the calls.
That is information you can find on your monthly bill.