I hope this may help someone else as its cured all my issues with sure signal.
I have an Iphone 5s and I found that the sure signal will only give me a 3G 5 bar
no dropped/failed call scenario when I switch off wifi on the phone.
I have now tested this dozens of times and it works every time. Wifi on and no 3G and dropped
calls aplenty, Wifi off and perfect!