Hows the Coverage outside the Gym?  have you tested the coverage that way if you haven't you might give it a try)  there's quite a bit of stuff between your Cell phone and the Towers outside your Gym)  Stone an or Concrete,  Metal in the Roof, glass many factors..  and if you had a tower that is close to the Gym that once had great signal and has changed it may have been changed to another Carrier..

But some things you can Try give customer service the ZipCode of the area of the Gym so they can check the towers in the Area, Have them check your PRL if it's out of date they can update it by sending it to your phone.. And if you have a Smartphone with a Sim Card you may need to either pull it "phone off"  leave it out for 10 min. re-install it see if that improve's the issue if not have the Sim replaced..

Hope that helps let us know what you find out.. b33