On a phone with voicemail switched off, Email2Phone calls back at 5, 15, 30, 60 and again 60 minutes later, five callbacks over roughly three hours, until someone presses 5, 7 or 8. After the fifth attempt the message is filed in your online Message History, where it can still be read.
A message is not counted as delivered because the phone rang, or because someone answered. It counts as delivered when a person presses 5 (reply), 7 (delete) or 8 (save) while it is playing. Picking up and hanging up is treated as unanswered, and so is listening to the whole message and putting the phone down. That is deliberate: the point of an alert channel is to establish that a human has it, and only a keypress proves that.
The condition matters. If the phone’s voicemail answers the call, the message is left there and counts as delivered, so none of the callbacks run. Voicemail and delivery covers that and what to do about it.
With more than one number on the account, a setting decides what happens once somebody acknowledges: stop there, or keep calling until every number has been reached. See ringing more than one phone.