When comparing Threema vs Linphone, the Slant community recommends Threema for most people. In the question“What are the best Skype alternatives?” Threema is ranked 10th while Linphone is ranked 14th. The most important reason people chose Threema is:
Registration is handled by random generated IDs. No phone number needed.
Specs
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Pros
Pro Registration without phone number
Registration is handled by random generated IDs. No phone number needed.
Pro Encrypted communication
Provides complete end-to-end encryption.
Pro Company behind Threema is based in Switzerland
Pro Servers in Switzerland
Privacy laws protect the data Threema has.
Pro Supports encrypted group chats
Pro Provides a web client
You can control and message with friends via Threema Web, the official web client for Threema.
Pro Ability to send pics and videos
Threema supports sending media messages such as video, pics and even voice messages.
Pro Group chat polls
Pro Allows verification of contacts identity via QR Code
Pro FOSS
Free and Open Source.
Pro Work well with FritzBox router
Pro Great SIP support
Basically every SIP provider supported.
Pro Simplistic and clean UI
Over the years, its UI has changed and improved by a lot.
Cons
Con Not free
Unlike many other options Threema is not free as it costs $1.99 for the app.
Con Single device only
You cannot use multiple devices to send messages. However, it does have message mirroring on a web client.
Con No cloud sync
E2E encryption means no cloud syncing of messages.
Con UI not very intuitive
Implementation of Contacts and missing calls are not very intuitive.
Splitted View (left Contacts, right Call-History) would be much better as current look&feel.
UI don't mind the "less clicks are better" philosophy. It looks a bit thrown together. Some Icons here, some there.. no navigation strategy are used (like known from webpages)
Con Limited to flatpack installs
Yet another container service to keep up with.
Con Doesn't ring on incoming calls
Even if the Audio-File exists and can be played in the Settings-Dialog, it doesn't ring for an incoming call. Worked some time ago, currently not.
Con No Plugins available to add external Address-Books for contacts
would be nice to be able to add NextCloud, GoogleContacts or other Cloud-Services, or just an local Address-Book (like Thunderbird)
Con Up-to-date version in 2020 can only be compiled from source
There is either an outdated version in Ubuntu repos, official flatpak install has been broken for over 6 months, so you're left with compiling from source.