@ThomaM e/OS queries and downloads from Google Play, so basically all Apps that are unrestricted (not needing a paid licensed or are geofenced) are available.
As Acrobits is paid and has no independent mechanism to buy and enter a license, it can’t be bought from an /e/ phone - but can possibly be installed and used after associating the /e/ phone with a Google Account that already has the App licensed.
On VoIP: I’ve had good experiences with [HOWTO] use the builtin Android SIP client to do VoIP and more so Sipgates Satellite (a service in Germany with a +49 national mobile phone number) - the latter uses Googles native push service (FCM) to signal an incoming call, that’s their secret sauce for establishing a call that is not present in the in-built client