That's the fun part. They did. DISA and the NSA have had the DMCC-TS (and DMCC-S) phones available across basically every agency or department for years now. They come already provisioned with Cellcrypt for messaging and phone calls as well as email access.

These are the standard communication device for senior members of the US government who are working regularly with classified information.

https://www.disa.mil/-/media/Files/DISA/Fact-Sheets/DMCC-TS-...

The issue with these devices is that the systems they use follow all recordkeeping laws and any communications are subject to FOIA.