I significantly increased the signal strength on my US GSM Ringo by adding a new, larger antenna. It is a Uxcell brand 700-2700mhz 6db model for $6.99 us dollars on Amazon Prime. It came with a smaller coax plug but I was able to reuse the stock antenna cable and plug. I cut the coax a little shorter to squeeze a bit more gain and re-soldered it to the new antenna. It fit it between the case layers above the RF module. I had to be careful not to short the copper antenna traces to the case screws and after chipping some case material out of the way from the solder connection it worked great!
Please see photo