It's more than pointless, a cellphone is EXTREMELY complicated, it would cost MANY times the price of buying one (assuming you could even do it, or get the parts). You then have the problem of actually constructing it, the high frequency radio sections would be exremely critical to build, and you would need lots of very expensive equipment to do so. Then you have the actual problems of the physical construction, phones aren't made by humans, they are made by robot machines, I doubt you have the precision required.