No, it's not. But the difference is in the caller pays model, there's no mechanism to reduce the inter-carrier cost. Retail cell phone carriers can hide that cost from their users, because balance of calling usually works out --- most users probably get about as many incoming call minutes from other carriers as outgoing call minutes to other carriers, so revenue from calls roughly matches costs from calls. For voip carriers it's different --- they can't charge other carriers a high rate for incoming calls (because voip is low cost), and they have to pay mobile carriers a high rate to terminate calls. So, retail cell phone users end up in the same boat of low cost "unlimited" calling, but business users have to pay 10-20 cents a minute (depending on the country and the tariffs allowed)
High cost is an effective anti-spam technique, but it also raises costs for legitimate business uses.
The mechanism is "regulation". EU intercarrier fees are limited to 0,4 cents per minute (lower for non-mobile networks), so they can not be a big part of the "10-20 cents" carriers might bill their business customers.
High cost is an effective anti-spam technique, but it also raises costs for legitimate business uses.