Multiple projects have proven that can be done, anyone who can connect outbound and use open peers to relay their stuff for them. Look at Gnutella for example that used to power Limewire, that could do it. But so can Tox, and Session uses a different mechanism with servers that communicate P2P with each other rather than everything being P2P.
Discussion
Yes, my point is, it can be done with a service like you describe. If all you have is a mobile device, a service still has to be run by someone who isn't you that has a port open to the internet and is willing to shoulder the burden of relaying your traffic between peers.