The Mueller investigation had basically bankrupt him. He lost his house paying his legal fees. In order to flip him against Trump, they threatened to prosecute his son. The prosecutors refused to provide Flynn's defense team with exculpatory information (which they are required by law to do). An attorney named Sidney Powell who specialized in federal appeals cases, reviewed Flynn's case and she took it on. Unlike Flynn's original defense team, she would not give up until the information he was due for his defense was legally given to him, and those files were just made public. Included were notes (I'm paraphrasing) "What is the goal, prosecute him, get he to resign, get him fired?" Other notes by the FBI agents who interviewed him said he did not lie, but Peter Strozk ordered the investigation to remain open and forged documents that led to his prosecution. There were about 30 other documents showing how Flynn was railroaded to make the Russia case against Trump, but good luck finding any mention of them in a google search. I tried to find it before I answered you and only found commentaries on why Flynn deserved to rot in jail.
In answer to your question, Flynn did not lie, he was doing his job when he called his Russian counterpart, among others from various nations, prior to the transition. He did not lie because he wasn't covering anything up.
Pray you never go to court for something you didn't do when the prosecution is the Federal Government with unlimited funds.