According to a recent ABA article, Lawyers who travel outside country may be asked to show the contents of their phone or laptop to a customs agent. A customs agent may not need probable cause so the 4th Amendment will not protect an attorney from searches at the border. The search can include their phone, tablet or laptop. The attorney could be required to open client files, reveal internet search history, show phone lists and phone logs. While it is true that if you tell the border agent that the information is client sensitive the border agent should not pursue the request,. But there is no guarantee the border agent will listen. All the border agent needs to do is state (s)he suspects a crime. This could mean to get your device back, you may end up disclosing confidential client information.
Also if a border agent asks for a password, the attorney should inquire whether it is a request or an order. Just giving the password up could be interpreted as a consent to search and an ethics violation.
Law firms such as Hinshaw & Culbertson require their lawyers to take burner phones and laptops when traveling outside of the country.