It needs a phone number. You can register your tablet with one phone number.(So you need a phone for the activation number)
However only one phone number per device. (Exception for the Signal WebApp for Chromium which you can pair with your phone/tablet)

More Infos on Signal in general you can find here: https://play.google.com/store/apps/d...rime.securesms
or on the official openwhispersystems homepage: https://whispersystems.org/