I am looking to reroute my calls on my iPhone to another mobile number

We are eliminating the 5 landlines in our office and using Iphones instead. The Norstar system we are using is cumbersome and not at all user friendly. I am wondering if there is an app, similar to the Newber App (that apple will not release in the iTunes store), available for use. It would allow an incoming call to be redirected to a mobile phone. After the call is transferred it does not take up the original line used for the call. As well, something that would have on hold music and an auto attendant would be nice. Thanks in advance for your suggestions!