HulloMail™ and other 3rd party voicemail providers use conditional call forwarding to divert incoming messages away from the carrier's voice mail system. All major carriers AT&T, Verizon and T-Mobile except for Sprint provide this feature for free and allow you to set it up yourself via your phone. 

Sprint customers are required to call customer service to activate this type of forwarding, and then it costs you 20 cents per minute.