The documentation says Speaktoo provides a REST API, so you should be able to use Mendix's Call REST action to call it.
https://www.speakatoo.com/documentation_view/speakatoo--api-documentation
If you are getting errors calling this API, try setting the REST Consume log level to TRACE. This will output the request from Mendix and the response from Speakatoo in your logs, which should explain what is going wrong.
https://docs.mendix.com/howto/monitoring-troubleshooting/log-levels/#standard-log-levels
Good luck!