Originally Posted by TwoSheds
Another answer may be that BB's are designed to lock-on to the strongest signal in the area you're in; it's unlikely that you'll get 3G coverage everywhere in the country (or even in certain buildings), hence the unit will search for the next strongest signal/transport medium (e.g. UMTS, GSM, gsm, GPRS, gprs, EDGE, edge etc. - the lower case versions meaning that all but calling and SMS functions will be limited or unavailable until you move to an area with better reception/signal strength).
Hope that helps.
SMS (Text Messaging) is available on all signal types and it uses the voice network to send information. MMS & Data is what is not available in "lower-case signal types")
The differences are: 4G/H+/3G:
Data & Voice services available, and can be used simultaneously EDGE/GPRS:
Data & Voice services available, however cannot be used simultaneously 3g/gprs/edge/GSM:
Voice services only available (includes the sending and receiving of SMS). Data Services not available. SOS:
Emergency Mode Only. Only calls to the Emergency Number can be made. Other Voice & Data Services are not available. X:
No Wireless Signal. No Voice (including Emergency) or Data Services Available OFF:
Wireless Radio turned off. Emergency Calls can still be made via the locked screen. All other Voice & Data Services are unavailable.
For more information: KB02334-Network status indicators from the GSM/GPRS/EDGE/3G/4G networks