iPhone experts, why am I getting this error message all day long?

[Grumpyguy]

The building I work in, has a dead spot, probably 20x20, right where I work. All my calls go directly to voice mail.

Anyway, after I returned my missed call today, I've been getting this message about every twenty or thirty minutes.

Nothing was or is attached to the phone when I got/get the message.

Thanks,

Bryan