The problem though is, using undocummented communication channels on private phones by people not technically inclined. That Signal is an american company and subject to NSA scrutiny while the users a politicians of a foreign goverment only makes this worse.
So, national messengers, controlled by experts, that archive communication and run on trusted hardware, would be the best solution for the work of democratic goverments I would think.
Of course, the possibility of software quality and security experts in service of the goverment is probably just wishful thinking.