We have recently tested a number of VoIP providers for use world wide within Enterprise Alert. Here are the results of those test and the VoIP providers that can be used with Enterprise Alert in different regions in the world.
GERMANY - PLACETEL https://www.placetel.de/ (Must have a address to use this VoIP. Will not function with an address outside of Germany - not even Austria)
USA / Worldwide - TWILIO https://www.twilio.com/ (There are issues getting this to function properly in Austria)
Austria - Innosoft: https://www.innosoft.at/ (Must supply a public IP address for the company to the provider.)
Sinch https://www.sinch.com/products/apis/calling/voice/ (Implemetation document attached.)
Other VoIP provider options (these have not been tested at this time):
3CX https://www.3cx.com/ (recommendation by NTT Data Austria) (not relevant as they do not offer phone numbers)