no connectivity error code

I need to know why I keep getting an error code. The error code is a circle with a line through it, and it says there is no connectivity. When this error code comes up I cannot connect to my service provider to make or receive a call. There is no problem with connecting with wifi. I can do a factory reset which will get rid of the error code, however I have to set up my phone again. What firm ware is affected with a factory reset? Thanks for any help at this point. Judy