President Donald Trump has continued to use his personal cell phone to make calls, despite repeated warnings from his staff that the practice could leave him vulnerable to foreign surveillance, multiple officials told CNN. The Democratic impeachment inquiry has resurrected concerns about the security and potential vulnerability of the President’s communications. Witness testimony revealed some top officials repeatedly failed to follow protocol intended to prevent sensitive phone conversations, including those involving the President, from being intercepted by foreign intelligence services. Several former US officials have told CNN it is highly likely that US Ambassador to the European Union Gordon Sondland’s cell phone call to Trump from a restaurant in Ukraine over the summer was picked up by intelligence agencies from numerous foreign countries, including Russia. Normally, a US ambassador talking to the President would do so from the embassy using a secure line, one former intelligence official told CNN. Cell phones, the former official said, were much more vulnerable than calls made at secure communications facilities. The lapse was only amplified by the fact that Sondland made the call in public, where it could have been easily overheard and in a foreign country that is already being targeted by foreign adversaries of the US, including Russia, current and former officials said. And the House Intelligence Committee’s recently released impeachment report includes phone records, which show a web of communications between conservative journalist John Solomon, Trump personal attorney Rudy Giuliani, Ukrainian American businessman Lev Parnas, the intelligence committee’s top Republican, Rep. Devin Nunes, and the White House’s budget office. Congress’ ability to subpoena Giuliani’s call logs from cell phone companies suggests that the calls were not encrypted through an application service like Signal or Whatsapp, nor were the calls were made on a secure line. It’s unlikely that Giuliani’s mobile phone was specially encrypted, and the logs indicate that the calls were not made via a secure line from an embassy. Even if the President was using a secured cell phone to call the former New York mayor, Giuliani’s unsecured line exacerbates security concerns. “All communications devices of all senior government officials are targeted by foreign governments. This is not new,” Bryan Cunningham, executive director of the Cybersecurity Policy and Research Institute at the University of California-Irvine, told CNN last year. “What is new in the cell phone age is the ease of intercepting them,” Cunningham added. “Of course, calls are only secure if both parties use a secure device.” The phone records do not identify Trump by name, but House Intelligence Committee Chairman Adam Schiff connected an “unidentified” number designated simply as “-1” in the logs to the President. But several members on the House Intelligence Committee told CNN that they did not know whether the numbers were traced to various individuals or who “-1” was. Since taking office, Trump has been cautioned against unsecured lines multiple times. But he has continued to call people on a number he held since before his time in the White House, including making calls from that number in the last several weeks, people familiar with those calls say. He also has a second number he uses, which is government-issued. On Friday evening, the President denied CNN’s report, saying he hasn’t used his personal cellphone in years. “I haven’t had a personal cell phone for years. Only use government approved and issued phones,” Trump tweeted. CNN stands by its reporting. These are cell phones Trump has personally been issued. They are considered to be less secure than landline White House phones presidents typically use, and he has been cautioned against using them to make secure calls, sources tell CNN. CNN previously reported that Trump has given his personal number to foreign leaders on multiple occasions, outright ignoring security protocols. According to The Washington Post, Trump handing out his personal line led to one-on-one calls that even top White House aides did not know about. The New York Times reported last year that Chinese and Russian spies have listened in on Trump’s calls on his personal cell phone and the President’s aides have told him that Russian spies were listening in on a regular basis. Senior officials view moving Trump away from unsecure lines as a lost cause at this point, one person said. For a brief period earlier in his term, Trump agreed to stop using his personal device, but quit after he realized all of his calls could then be monitored on a call log list. Former White House chief of staff John Kelly had staff monitor the list routinely while he was still at the White House. UPDATE: This story has been updated with more information about the cell phones the President uses.