That's a tough issue, unfortunately, and a common cause for complaint.
Many cell phone carriers advertise "no roaming charges" when in fact there are roaming charges defined in microfine print. Perhaps other users can expand on this and how to handle it.
I guess the first thing you should do is check your contract and service plan for its definition of roaming and your local territory. If you were not in roaming territory you shouldn't pay the bill.
Otherwise perhaps you could negotiate a reduction in rates. Maybe others have better suggestions.
[My phone is pretty explicit -- only my immediate region is local, everywhere else is roaming at $1.00 minute surcharge. But a little light on the phone goes on when roaming. I have noticed that in border areas (e.g. a rest stop on the turnpike at the edge of my service territory), that sometimes I'm roaming, sometimes I'm not; because the base site antenna used varies all the time even in a fixed location.]