[dmix]

You also don't need to use these private numbers with friends, keep your main number and just use this or some voip number for stuff you want private.

The problem is this would both a) need to be used by Signal as the primary number ID b) your friend would need to know this separate number just for chat (unless you also use it for calls and other stuff then no biggie)

Of course this also kills new friend discovery, which is probably not needed by most.

I haven't tried using SIP on my phone app while simultaneously having a SIM number so I'm not sure Signal can be routed through an arbitrary number on your phone. But I could be wrong. Maybe someone else knows?