There's a service I contacted a while back to help me find a T1 provider for our new office.
www.ShopForT1.com There are T1 providers that have dual-use data/phones, that are scalable. The voice part of the T1 can be broken out to loop start lines with a channel bank.
The plans they have available all had some sort of huge "included" minutes.... But it's not going to be extremely cheap, but better than paying Verizon for each individual POTS line.
As a software developer for an automated/computer dialer company, I can tell you to stay clear of consumer-grade VoIP services like Vonage or Optimum Voice. They have known issues that affect PBX's and dialers, when calls are disconnected.