Have you tried other ways of getting a hold of the dev? I feel you must talk to him somehow. Get a friend to phone him from a different number.
What will you do now, going forward? Does this mean you're giving up your business idea?
I have tried different means, however I not going to continue pursuing phone calls as it clearly does nothing.
By no means have I given up, I am working with a stunning developer and within hours of receiving my requirements, he published a demo for me containing some amazing functionality. This business idea is for from over ;-)