The last time that I checked with our telcom staff, they said access to the API's would cost us $$$, and it was too much for us.
HACK ALERT: I discovered I could set up free accounts on IFTTT and build an IFTTT rule so that an incoming email results in an outgoing phone call. Not pretty.... and very brittle. And setup/support of IFTTT accounts is time consuming. However FREE and works well for us. But if you need to support more than a couple dozen target phone numbers, then I would never consider this option.
BONUS HACK: We also use email to send text alerts to our staff. Every cell phone carrier accepts text messages via email. e.g. ##########@mms.att.net for ATT . We just ask that staff type this entire string into our repository.