alt Hacker Newspeople point out in sibling comments that is phone call then be out of client-attorney privileges? since it goes through a "3rd party"? maybe not the call itself but the voicemail for example. can it be "extracted" for the same purpose? another point to make it safer would be sharing the "chat" with the lawyer, this way it becomes media of communication.
Again it comes back to the three elements of privilege: between an attorney and client; kept confidential; and for the purposes of legal advice. So for a phone call, I think that holds. There's a reasonable expectation of privacy on a phone line. I should note I'm not a lawyer but a SWE and I looked at Heppner closely RE: the privacy angle.