Hi Dennis,
I would look at https://www.twilio.com/docs/voice/api/sip-interface, from an architectural point of view, this should work. Mendix is great in REST API handling.
A second option is to use a second telecom specialized Low-code platform. It supports all common Telco integrations and can export the function using REST for you to consume in Mendix. https://www.ect-telecoms.com/thats-what-we-do/low-code-for-telecoms-services
The solution and its costs would justify looking more into the challenge you have. Maybe these directions are already sufficient, if not, please share more details of your usecease.
Go Make IT