@sigmund were you able to get clarification on this? If so can you share that with us? I have also included the link to the Dynatrace Supported Technologies: https://www.dynatrace.com/support/help/technology-support/supported-technologies-and-versions/
If they are using the Infobip SMS and/or 2FA API, you should see the outgoing requests in Dynatrace under the "Requests to public networks" service. From there, you'll be able to tell Dynatrace to monitor infobip.com (all requests going to that domain) as a separate 3rd party service.
To get an idea of good candidates for key requests , check the Infobip API documentation here: https://www.infobip.com/docs/api. To monitor via synthetic, check the API service status docs: https://www.infobip.com/docs/essentials/api-service-status.