open source for call center compatible with CiviCRM
According to Alternativeto.net, there are no open source alternatives to Callhub.  Arguably Asterisk VoIP is an alternative, but you would have to add a lot of software on top of it.

Regardless - while Callhub has CiviCRM integration, there are no alternatives that already have CiviCRM integration.  You would have to create it yourself.