Why British staffs kept phones and runners

Did signals intelligence help explain why British staffs continued to use phones and runners where possible?

Yes; Britain knew the danger because it was exploiting other countries' signals itself.

British use of telephones and runners where possible was not simply technological conservatism. British analysts were exploiting other states' communications and understood that the same methods could be used against them. Wired or hand-carried messages reduced the radio traffic available for interception and location. Communications practice reflected institutional knowledge of signals intelligence.