Not only could this offence be impeachable, it could be high treason.
At the very least, perjury:
"Sally Yates, the acting attorney general, judged an intercepted call “highly significant” and “potentially illegal” under the Logan Act, which bars private citizens from interfering in diplomatic disputes with other countries. When word of the Flynn-Kislyak call leaked on Jan. 12, a Trump official denied that sanctions were discussed. The White House spokesman, Sean Spicer, gave a similar answer on Jan. 13, as did Vice President Mike Pence on Jan. 15. After Mr. Trump’s inauguration, Mr. Spicer said on Jan. 23 that Mr. Flynn again assured him that sanctions had not been discussed. Shortly afterward, Ms. Yates, with agreement from James Comey, the F.B.I. director, informed Donald McGahn, the White House counsel, about what really happened."