How-to: Is there any difference between USA and Europs signals?

Hi there,

I got two cell phones from AT&T when I was in USA (Maryand) and they worked OK the time I lived there. After a year a went back to Spain so I unlock the phones to use them here. IThey work but I can't get a steady signal from my supplier. I got the four bands in both cell phones, so it is not a problem related with bands, but maybe the cell phone are ready to get the signal from a different region of the band.
Any idea about this? Any suggestion about how can I get full signal? The cell phone is Sony Ericsson W350a, and the pother in LG neon.

Thanks

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