Exclamation Low Signal on UMTS please help

When I connect to UMTS the signal goes too low nobody can call me, but when i connect to EDGE i have 100% signal
please help and answer soon as possible it is a big problem


My phone: Samsung Corby Smartphone -------> Sony Ericsson Live With Walkman(wt19i)----->Sony Xperia P (LT22i)--------> Samsung Galaxy S3
New in the android world so please help me