I was wondering if anyone can explain how cell phone apps like "Sideline" or "Text Now" are able to offer users a choice of a second free number for use. How and where are these numbers managed and claimed by these applications for our use?

The Sideline app uses your cell phone service provider and is the more interesting example of second number use vs the VoIP app. Unfortunately, it is no longer free.

See More: Applications offering free second numbers