How can Vivendi have this much influence over TIM without owning it?

Following the removal of Flavio Cattaneo as CEO, the telco has appointed Arnaud Roy de Puyfontaine, the current Vivendi CEO, as interim boss.
The last couple of months have seen Vivendi wrestle more control of the telco without increasing its stake, which currently stands at 24.9%, just below the levels required to initiate a takeover. The board is already filled with Vivendi friendlies, and some might have assumed Cattaneo was a puppet appointment with de Puyfontaine pulling the strings. Now it