2 iphone 4s', 1 updates other won't

I have 2 iphones, both are the 4s and run on the same carrier.

1 updated to 'iOS 9.1.The other one says ' 'iOS7.' (not sure of the # after the 7.)

When ever I check for updates it comes back with 'Your software is up to date'

Also this same phone is always cutting out during a call and I even get dropped calls sometimes.

I would like to get phone updated to iOS9.1. Is there some setting or can I 'force' an update?

Thanks