Callhub is a good app that gives you call center functions without committing to a lot of seats. They charge per minute instead of licenses. This provides you with a pretty good app without commitment, and allows you to scale up or down as needed/ Review collected by and hosted on G2.com.

The cost per minute is on the higher side, and the reporting features could be improved. It is still very B2C centric, and not as optimized for B2B campaigns Review collected by and hosted on G2.com.