Doesn't zoom also support h.323 and SIP[0]? There are open client available for this [1]. Don't know how good these actually work.
[0] https://support.zoom.us/hc/en-us/categories/200110033-H-323-... [1] https://www.gnugk.org/h323-endpoint.html
[0] https://support.zoom.us/hc/en-us/categories/200110033-H-323-... [1] https://www.gnugk.org/h323-endpoint.html