There is no umpire in volleyball! There is an R1, otherwise known as an up ref who calls all the rules.
Coach or umpire
The umpire assists the referee.
umpire scorer referee
a volleyball a net two teams and a umpire and a crowed and a uniform
a umpire is in baseball and a referee is in other sports such as football (american), Soccer, volleyball etc..,
you have to call the balls that hit the line or go out when the ref can't see
In volleyball, the umpire is sometimes called the second referee, R2 or down official. His duties include watching the net, team alignment on service, back row attacks, manage the coaches, substitutions, score sheet and to generally help the R1.
The umpire assists the referee. If you need two people( like I do... :D ), I don't know the second person.