All you Qs should be addressed to your VoIP provider. Some allow phone extensions or sub-accounts setup via their website, so your Android Zoiper will answer after a desktop version or home phone is not answered for a set number of rings. Availability and number of such extensions (free or paid) depends on each VoIP provider, but your phone number usually remains the same for all ext, and they all are registered at the same time on the provider's server, meaning that dialing an extra ext number is not required, transfer is done automatically based on the rules you set in your acc. Your mileage may vary depending on the provider.
As to using a VoIP home phone and desktop versions at once, since they are usually connected through the same IP assigned by your internet provider, the chance is only the last connected device will ring, unless you assign separate exts for both. Meaning, when you start Z on desktop, the home phone won't ring anymore, since its registration with VoIP server will be superseded by the desktop app's. Again, YMMV.