I don't think you ever need it again, but if someone else later owns the number, they could register it with Signal and you would no longer be able to send or receive Signal messages.

>For registering, is it possible to use a disposable number?

Yes, I use a Google Voice number for signal. It's theoretically possible to use something like a payphone or a free inbound number from callcentric, so long as you can receive incoming calls.

It's not required again, though if someone takes over the number they can re-register on Signal. You'll stop receiving messages then, and your contacts will get a warning that your safety numbers changed.

You can use Wire's API (in alpha) without a phone number, https://github.com/wireapp/wire-bot-java && https://wire.com/en/privacy/

Wire is not Signal, right?

Their protocol is derived from Signal's. I don't know how close it is. Maybe just the name, because Moxie does not like independent reimplementations.

Wire also got a favorable audit recently. https://techcrunch.com/2017/02/10/messaging-app-wire-now-has...

Signal requires a phone number, Wire does not.