According to a friend who used to have prepaid service on Alltel, the
prpaid service goes through a different network that somehow gets
piggybacked onto the main service provider's network.  And this backdoor
network is always overloaded at peak times - like during the first hour
of unlimited service at night, or on holidays when people want to make
all those calls for free.

Although they do get money from customrs in advance, they are not the
type of customers that companies like Verizon can count on to be around
from month to month, and so no company spends the money to keep the
system up to the potential demands at peak hours.  Of course, they don't
tell the pre-paid customers that they are second or third class citizens
when it comes to cell phone service.