iPhone data connection types
I sometimes get a "o" icon instead of the "3G" or "E" icons when I loose my connection to the network. Why?
Could this be due to a jailbreak?
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You are connected to GPRS/1xRTT since other network connection types are not available.
iOS 5.1 status icons
On GSM (All but Verizon):
On CDMA (Verizon):
(iPhone User Manual on Page 12)
It means you are using the slower GPRS connection. Probably because there's no 3G/EDGE connection or the signal strength for 3G/EDGE is very weak at your location.
Nothing to do with your jailbreaking.