Phone antenna

I made the mistake of buying a cheap Chinese phone on Ebay
(don't buy phones from Shaosan Li)
The reception was useless and in only worked in direct line of sight of a phone tower less than 50 m away.
With nothing to lose I pulled the back off the phone and located the internal antenna, and using the battery I jammed a loop of insulated copper wire against it.
It now works a little better - Great outdoors but only sms indoors.
Is there someone here that can save me weeks of trial and error and tell me the
optimal length / thickness / material I should use to improve reception even more ?

I have an advice to solve faster this problem. Drop that phone to the garbage and buy other one.