You are using a SMS/FAX provider the Communicate Extension of Workflow Designer do not support? That’s no problem if you could develop your own PHP interface. You also could ask our team if we should do this for you.
You need to add one file to folder modules/Workflow2/extends/communicate with the filename <individual>.inc.php
The structure of this file have to be:
[prism field=providerfile language=php]
This will be enough to add your custom provider.