Erro de chave Linphone (Ubuntu 16.04)

Instalei o Linphone via flatpak em várias máquinas com Ubuntu 18.04 (ou distros nele baseados) sem problemas, seguindo estes passos simples:
~$ sudo add-apt-repository ppa:alexlarsson/flatpak
~$ sudo apt update
~$ sudo apt install flatpak
~$ flatpak --user install --from https://linphone.org/flatpak/linphone.flatpakref

Ao tentar instalar o Linphone em máquinas com o Ubuntu 16.04, seguindo os mesmos passos descritos anteriormente, dá o seguinte erro:

erro: Falha ao instalar com.belledonnecommunications.linphone: Commit bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441: GPG signatures found, but none are in trusted keyring

rodei o comando para instalar com a opoção --ostree-verbose e ei o que ele me mostra:

~$ flatpak --user --ostree-verbose install --from https://linphone.org/flatpak/linphone.flatpakref
F: Adding GPG keyring data from linphone-origin.trustedkeys.gpg to verifier
F: Adding GPG keyring data from flathub.trustedkeys.gpg to verifier

com.belledonnecommunications.linphone permissões:
ipc network pulseaudio x11
devices file access [1] bus ownership [2]

[1] xdg-desktop, xdg-documents, xdg-download, xdg-music, xdg-pictures, xdg-videos
[2] org.linphone.SingleApplication


1. com.belledonnecommunications.linphone x86_64 4.1.1 linphone-origin < 120,0 MB

Prosseguir com essas alterações para a instalação de usuário? [Y/n]: Y
Instalando… F: Preparing transaction in repository 0x558ac80da200
F: Pushing lock non-blocking with timeout 30
F: Creating repo lock table
F: Opening repo lock file
F: Push lock: state=unlocked, depth=0
F: Locking repo shared
F: Using new tmpdir staging-560361b3-b17e-4470-b0a7-ce12fa90ebea-a4dz5N
F: Loaded linphone-origin summary from cache
F: Adding GPG keyring data from linphone-origin.trustedkeys.gpg to verifier
F: starting fetch of bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441.commit (detached)
F: fetch of bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441.commit (detached) complete
F: starting fetch of bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441.commit
F: _ostree_fetcher_should_retry_request: error: unset, n_retries_remaining: 5
F: fetch of bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441.commit complete
F: Adding GPG keyring data from linphone-origin.trustedkeys.gpg to verifier
F: _ostree_fetcher_should_retry_request: error: 544:2 Commit bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441: GPG signatures found, but none are in trusted keyring, n_retries_remaining: 5
F: pull: idle, exiting mainloop
F: Aborting transaction in repository 0x558ac80da200
F: Popping lock non-blocking with timeout 30
F: Pop lock: state=shared, depth=1
F: Unlocking repo
Erro: Commit bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441: GPG signatures found, but none are in trusted keyring
F: Free lock: state=unlocked, depth=0
F: Closing repo lock file
F: Free lock table
erro: Falha ao instalar com.belledonnecommunications.linphone: Commit bae79f29ce390b6b28254cba0ae63f1bbb8d9a30f50acce422ce5ef1f1839441: GPG signatures found, but none are in trusted keyring

Fui verificar no site da linphone e vejam no link o que encontrei https://www.linphone.org/snapshots/ubuntu/

Só tem coisas para o 18.04!

Linphone só dá suporte ao Ubuntu 18.04?

Se não, como resolver esta treta?