According to this article from Labor Notes, a decision made by the National Labor Relations Board in February of this year determined that, “an employer cannot enforce a rule banning secret recordings if employees are using their phones to further their group interests or to lay the ground for forming a union.”
According to the decision, “In many instances, workplace recordings, often covert, have been an essential element in vindicating employees’ Section 7 rights.” (Section 7 of the National Labor Relations Act guarantees organizing rights.)
Section 7 of the NLRA guarantees workers “the right to self-organization, to form, join, or assist labor organizations, to bargain collectively through representatives of their own choosing, and to engage in other concerted activities for the purpose of collective bargaining or other mutual aid or protection,” as well as the right “to refrain from any or all such activities.”