Hi Adit,
Ur name sounds like u r frm India too. Back to ur question,
SOS Calls are some special no.s which u can always dial even if there is no balance in ur mobile or ur validity has expired. Like, some default no.s stored for SOS calls are 911 etc. which is no. of cops in America. In our Country, I've never used this feature. And I'm pretty much assured that it'll also not work here, as once Validity or cash balance gets exhausted, mobiles turned into a PAPERWEIGHT here.
Sometimes, even if ur cell is working fine, u'll enter in some conjested locations, basements, tall buildings, hospitals, army cantonment areas and highly secured places where there are Jammers installed. In any of the above conditions, ur mobile cannot receive the network, so, displays shows "SOS Calls Only".
For example, in my case, my workplace is installed with jammers, so, occasionaly, my mobile shows this signall.
Also, if the cell phone which u r using have some internal problem with the network reception, chances are there to get this msg very often.
Hope i've made the concept pretty clear. Bye.