One of the problems that sometimes arise when using the Blackberry is a low-signal indicators or small, and sometimes even display the SOS or your Blackberry is not connected to the network operator. If you ever have problems like this, the first step to try is to change the sim card from another operator, to ensure that the problem is not caused by signal operators or signal range of factors that are less than the current service is used. If after using other operators, Signal Blackberry is still small / low and sometimes SOS, then it is certainly a problem on the Blackberry.
The second solution, you can try to change the Network Mode is used, if the previous use 3G & 2G Network Mode, then try to change just by using 2G. As we all know the 3G signal is not evenly in all places as compared to 2G which has been ascertained at all place is definitely within reach of its existing 2G signal. Also try to change the Network Selection Mode is also on it from Automatic to Manual.
Some of the alternatives if you are having signal problems on the Blackberry is trying to do the Hard Reset, while the Blackberry you're on, unplug the Blackberry battery for 30 seconds or so then plug it again, and wait until the Blackberry is finished booting. Hard Reset will not erase the data on the Blackberry.
Low Signal Problem Solution Blackberry and sometimes SOS