And you haven't even mentioned the fact that mobile phones can move around, leading to high rates of dropped connections as devices come and go. There's no way this is going to work.
Sure, it's not as nice as a WAN connection, but the average cellular contract is pretty expensive per month. Something like $10 per GB, and often a $30 and up base rate.
So sure, long distance multi-hop mesh stinks for real time voice, but could be quite usable for other use cases.
Also many common delay tolerant network implementations rely on message replication to increase the probability of delivery of the message. This puts additional bandwidth strain on the inter-node hops of the network, which as some of the other commenters pointed out, not actually all that high.
I think specifically with voice it should be possible to send two chunks such that if they both arrive then you get your audio, and if only one arrives then you still get your audio but at a lower quality.
This is the kind of thing you do a test with in a specific targeted community though...