How to report a match?

A not too sporty player, taking advantage of a loss of connection due to a phone call, claimed the victory as he was about to be beaten by me (he had already lost the queen and we were close to checkmate).
How do I report this and restore a more honest score?
This is the game, you judge ...

Things like this happen. There is no way you can report him for this. Just make sure you have a good connection. It has happened to me many times before.

Although you were in a winning position - you chose to answer the call. That was your decision. The other player did not break any rules. And lichess certainly cannot "restore" a score. Good luck with your future games.

Lost connection = lost game
If you lose connection, you have some time to re-establish it.
If you fail to do so, you lose.
Neither lichess nor your opponent can know why you left the game.
In over the board chess leaving the playing venue is forbidden as well.