Why aren't more VoIP carriers offering pre-paid minutes?

CallWave Inc., which allows customers to redirect calls from a cell, VoIP or landline number to a designated number, is debuting a pre-paid service this week. This service has a follow-me-home feature that transfers calls from switched-off cell phones to landlines (including VoIP). The service is $3.95 a month, plus a one-time $19.95 charge if the transfer is made from a Nokia phone.

Sounds reasonable, but while this plan has a pre-paid VoIP component, it is not really pre-paid mobile VoIP. My idea for pre-paid VoIP would not necessarily have to be a SkypeOut model, but a plan where Vonage, Packet 8, Verizon, or just about any other VoIP provider would let you buy a given number of minutes rather than enroll in a monthly plan.

The upside is marketing-related, but one that also serves the customer.

Pre-paid VoIP minutes would be a good introduction to VoIP for those who might want to tip their toe into the water - but don't want to buy the requisite equipment, pay a connection fee, and then sign up for a monthly plan.