Can CiviCRM be integrated with a VoIP system?
The closest we have in our docs is integration w/ SMS as per:
https://docs.civicrm.org/user/en/latest/sms-text-messaging/set-up/

This documents the integration w/ Twillio and Clickatell.

At the extension level, you'd find a few here: https://civicrm.org/extensions?title=&amp;body_value=phone&amp;field_extension_civi_use_target_id=214&amp;status=All&amp;field_extension_ready_value=All

You probably want to play around with the value in the description field (I've got "phone" in there). There are maybe 6 or so extensions that provide some level of integration w/ external systems.

The best advice is probably to narrow down the scope of your question, and focus on what you are trying to achieve rather than a general question about technology that can be used for various different purposes. "Integration" is primarily a word for sales and in real life can mean many different things.