Allow VM Notify to call a number upon delivery failure

Currently the Voicemail Notify module calls the list of recipients in the following pattern (assuming a list of only 2 priorities):

call priority 0 users once
wait for 'priority delay' timeout
call all priority 1 users once
wait for 'retry delay'
call all priority 0 users again
wait for priority delay
call all priority 1 users again

repeat the pattern for "retry count" times:

User has a request where they are able to call the "Big Boss" once at the very end if the delivery attempt fails. This could be achieved two ways:

1. add an option to call a number on failure as is done for the send email on failure
2. add a toggle that changes the calling pattern. Instead of current pattern, call all priority 0 retry times, then call priority 1 retry times.