said by Stewart:For best reliability, choose a provider that is their own resporg, so your calls can be quickly rerouted in case of a carrier outage.
OP, if you don't happen to know what a RespOrg is, they are folks who actually control a toll-free number, they play a similar role to a CLEC/ILEC for a regular phone number.
CallCentric is a RespOrg through their Telengy subsidiary.
Vitelity is through their VCLS unit.
FlowRoute is as well.
Not sure about the others (up or down).