Former US National Security Adviser Michael Flynn is cooperating with the Special Counsel Robert Mueller to determine whether Donald Trump's associates colluded with Russian government officials during the 2016 electoral campaign and also in the two months before president-elect Trump assumed office on January 20th. Flynn has pleaded guilty to lying to Federal Bureau of Investigation (FBI) special agents regarding two late December telephone exchanges with former Russian Ambassador to the US Sergey Kislyak.
The first call was initiated at the request of Trump son-in-law Jared Kushner and related to Russia's possible use of its United Nations Security Council veto to stop a resolution condemning Israel's illegal settlements, as the Barack Obama Administration had decided to abstain to send a message of disapproval to Tel Aviv. If Russia had agreed, which it did not, it would have meant conniving with Moscow to do something sought by Israel and opposed by the elected government still in power in Washington.
The second call possibly was requested by Donald Trump himself and was made while Flynn was lying on a beach in the Dominican Republic. It sought Russian agreement not to escalate the tit-for-tat expulsions of diplomats that had been set in motion by the outgoing Obama administration. Russia delayed any possible expulsions, eventually implementing them when the Trump administration proved unable to mitigate other sanctions put in place by Obama. Both phone calls took place after the American election. Neither had anything to do with possible collusion regarding the election.