Does voicemail count as a delivered alert?

Yes, and it is the one setting worth getting right. If the phone’s voicemail answers an Email2Phone call, the message is left there and counts as delivered, so the callbacks never run.

Switch voicemail off on any number that takes an alert somebody has to act on. With voicemail off, nothing counts as delivered until a person presses 5, 7 or 8, which is the whole point of the callback ladder.

The same rule reaches a list of numbers. If the account is set to stop at the first acknowledgement, a voicemail pickup on the first number ends the sequence before the second one rings; see ringing more than one phone.

How you switch it off depends on the carrier or the phone system, not on Email2Phone. If a particular line will not let you, add a different number to the account and point the alerts at that one instead.

Voicemail on a phone that only receives low-priority notices is fine. The rule is for the number that has to wake someone up.