Have you investigated third party calling providers such as Aircall or Cloudtalk? If you click into that link, you can see a list of external providers currently supported by our API. Any user with a sales starter seat can make calls using these providers in their HubSpot account. I can't speak to if they specifically support SIP trunking, but that should give you a place to start.
If you've already considered these options and they don't work for your use case, please reach back out and I'll do some more digging!
We are currently checking the third-party integrations available in Hubspot Marketplace, however, usually they are tied to using services of the corresponding providers.
Our company has a solution that we have used for a few years now, and it utilizes SIP trunking. It would be great to just enter SIP account credentials and start calling our customers using the system we have in place.
Do you know about any other integrations/solutions that can help with that?
Thanks for the additional context. Would it be feasible to use your internal development resources to create a custom calling integration using the Calling Extensions SDK? As that could be another potential solution for integrating with your existing system.