Who's done it and what advice do you have?
2 Answers
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=&body_value=phone&field_extension_civi_use_target_id=214&status=All&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.
having the same question by now Not done yet, but will check http://docs.bigbluebutton.org/ and other open source gateways (+asterisk etc)