Just create a second group in signal with your friends called "group X: call only". Then everyone mutes the group (which I think mutes calls as well). Then when

Just create a second group in signal with your friends called "group X: call only". Then everyone mutes the group (which I think mutes calls as well). Then whenever someone wants to join the "voice chat" they can just initiate a call. If someone else sees and joins, so be it.