With the low cost default value, SMS are sometimes received many hours after. For a monitoring software, this has no sense, we need to be sure to receive the alert by SMS instantly to repair the problem as quick as possible. Otherwise, SMS is useless, email is free and good enough. Moreover, the price difference between low cost and premium is small.
I noticed this and the nexmo one were renamed as part of 1123603f64 for [refactoring] PSR-4 support through composer, mass-renaming all *.class.php to *.php to adhere to PSR-4
I was looking at the nexmo one as a follow up to Issue #170 Implement Nexmo SMS
See also...
https://github.com/phpservermon/phpservermon/pull/436