https://www.bulkvs.com/orig.html
$0.06/mo. + $0.0.0003/min. I have no experience with them.
https://signalwire.com/pricing/voice $0.08/mo. + $0.00325/min.
They give you a small credit at signup, so you can test without making a payment. IMO reliable, but they don’t have good failover options for when your PBX is down or unreachable.
http://www.anveodirect.com/prices/did
$0.15/mo. + $0.004/min.
Can fail over to an alternate SIP address or a PSTN number.
All of the above are pay-per-minute with no charge for channels.
https://www.localphone.com/incoming_numbers/united_states $0.99/mo. Hard limited to two channels, but no charge for incoming calls to VoIP.
Can fail over to PSTN or voicemail.
https://www.voxbeam.com/dids/united_states
$0.50/mo. Hard limited to two channels, but no charge for first 10,000 min. incoming calls to VoIP.
Can fail over only to an alternate SIP URI.
The above are two brands of the same UK company.