President Donald Trump is breaking with tradition by visiting Poland, an ex-communist country in central Europe, before making a presidential visit to longtime allies Britain, France or Germany.
Moscow has only once before suspended the hotline, which was established in October 2015 to prevent conflict between the different forces operating in Syrian airspace.
Hundreds were arrested, including opposition leader and protest organizer Alexei Navalny, who was seized outside his Moscow residence while heading to the rally in the city center.
Poland and Hungary have refused to take in a single person under a plan agreed in 2015 to relocate 160,000 asylum-seekers.
Eastern European countries including Hungary, Poland and Slovakia have flatly opposed an EU plan adopted in 2015.