It seems possible if you have (another) phone number
According to Wikipedia on Signal (the new name: "In November 2015, the TextSecure and RedPhone applications on Android were merged to become Signal for Android."):
Signal requires that the user has a phone number for verification.
The number does not have to be the same as on the device's SIM card
[...]. A number can only be registered to one device at a time.
How to register
Basically the FAQ states you can register with a different number:
Request a call for the verification number after the two minute timeout.
(Which phone number is shown as sender is also explained in the FAQ.)
How to build from source (& install)
According to an old issue on their site, it was only three commands build and install from source:
adb install -r bin/TextSecure-release.apk
How to download APK (& install) - this maybe risky(!!)
According to this Github-issue:
multi-device support is on the roadmap
But in the meantime, one can try downloading the APK from other websites, e.g. (same issue):
You can try sideloading the app via https://apps.evozi.com/apk-downloader/.
This is a user response, I do not know if/how trustworthy this site is!!! But, based on the present Playstore-link for Signal that link would be: https://apps.evozi.com/apk-downloader/?id=org.thoughtcrime.securesms .
Why no APK-download or ...store
I do not know, why Signal does not allow installing to a device without SIM (yet), and they may (see quote above). I do know, why they did (do?) not distribute their APK-s for 'wild' installation, see this issue:
- No upgrade channel for effective security
- No app scanning (possible malware problems)
- No crash reporting
- No stats to prioritize development and platforms
- Not a privacy win
- To not encourage "allow 3rd party APK-s" by 'naive users'
- It is still possible to install by building from source for 'power users' (only)