There are several cloud based SMS providers that we have tested and confirmed to work with Enterprise Alert. The most familiar is ClickSend as we have this running in our environment as well as a few other customers using it.
ClickSend -> https://help.clicksend.com/Integrations/derdack
SMSGlobal -> https://www.smsglobal.com/
Aerialink -> http://www.aerialink.com/
2SMS -> https://www.2sms.com/
HSL -> https://hslsms.com
Sinch -> https://www.sinch.com