iPhone 4s dropping calls

[retsub]

I'm in zip code 50036 and my 4s is now dropping every call after I receive the ring and answer.  Sometimes I might hear the first word or two before the call just quits.  This started a week or two ago but never happened in the previous 2 years I've had the phone.  I only have 2 dots of service on the indicator, but that's no different from earlier when the calls were okay.  Also, the phone works fine at my work location in 50010.

I've tried the network reset on settings, but it has made no difference.