"if blocking cellular signal is infeasible" - just get them signed up with AT&T, ZING! On a more serious note, have you tried working in a public library rathe

"if blocking cellular signal is infeasible" - just get them signed up with AT&T, ZING! On a more serious note, have you tried working in a public library rather than a coffee shop? Just ask the librarian to hold onto your phone while you're there.