You can check this SIP client for Nextcloud: SIP Trip Phone.
It can be connected directly to the SIP provider, if the provider allows direct connections from WebRTC applications, or it can be connected to the provider via Asterisk. You can install Asterisk on your VPS, alongside Nextcloud, then connect Asterisk to a SIP provider like Telnyx or Localphone, so that you can use a real phone number located in a country of your choice, acquired from the SIP provider, to make and receive phone calls to/from any phone number in the world.
It has complete documentation on how to configure it, as well as on how to install Asterisk, which can be used to implement advanced PBX features, such as: voice menu, voicemail, queue management, music on hold, number blacklisting, call recording, audio conference calls, etc.
If you want to send/receive SMS messages and faxes in Nextcloud, please see SMS Relentless and Pax Fax respectively.
If in addition to audio phone calls you need video calls, video conferences and text chat, you can use Roundpin, but this is a standalone application, independent from Nextcloud.
Feedback is welcome.