Why has my Galaxy S4 been dropping calls like crazy lately?

Why has my Galaxy S4 been dropping calls like crazy lately?
Added (1). I have it through Straight Talk. It used to never drop calls but lately it has been doing it but only at my house and it doesn't matter where I am at in my house or even if I am outside. I will go to answer a call sometimes and it will say I have 2 bars of service I answer and it drops to one bar of service and eventually drops the call. My service driving also seems to fluctuate between 2, 3, 4 bars in short distances. Is there something wrong with my phone or is this a service issue?
Added (2). My carrier said that they had solid coverage throughout the area.

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