You call a friend on the phone and repeatedly get sent to voicemail, so you continue to call her every 5–20 minutes hoping to speak to her personally. which reinforcement schedule is this?

Variable Interval

Explanation:

In a variable interval reinforcement scheme reinforcement is presented when the body issues at least one response after a variable interval after a variable time interval. An example of this is happening in the above question, where a person calls his friend's phone every 5-20 minutes and the friend does not answer, but the call ends up on voicemail.