Why not go with bare metal from say OVH for this type of service? Running a free VoIP service on top of a cloud provider is kind of silly, not just from a cost perspective, but also having shared resources for your voice/WebRTC media servers. But then the question is, why offer free VoIP service to begin with? You're going to invite a lot of abuse and fraud attempts very quickly. Sure you can guard against that, but is it really worth it?

There's a massively booming VoIP industry right now (despite the perception of "too many players" in the market). Go out there and make money while providing good affordable service. Charge $10-$15 per line while the big guys ask for $25-$50. VoIP seats add up fast, and it's very nice MRR with not a lot of hassle if you run things right and give customers tools to get things done without calling for support for every little thing.

Or go the route that Dialpad is taking with their limited Free product (dialpad.com/free) which is kind of a bait-and-switch because surely a business will need to convert to a paid subscription of some sort if they don't end up going out of business in the first few months.