There have been multiple reports on how Trump communicates via unsecured lines, especially among his inner circle, because using secure methods is too great an inconvenience. Intelligence analysts believe Russia or other foreign powers could use these conversations against both America’s foreign policy and Trump individually, and have detected signs of such spying.

Particular concern has been raised about the Kremlin listening in on conversations between Trump and his lawyer Rudy Giuliani regarding communications to the Ukrainian President over military aid for defense from Russia. White House officials have stated that Trump makes such calls regularly.

Politico – ‘Too inconvenient’: Trump goes rogue on phone security
The New York Times – When Trump Phones Friends, the Chinese and the Russians Listen and Learn
The Washington Post – Signs of sophisticated cellphone spying found near White House, U.S. officials say
Independent – Trump ‘routinely’ made sensitive calls on lines open to Russian surveillance, White House officials say
The Washington Post – Phone logs in impeachment report renew concern about security of Trump communications