Call Forwarding: Who Gets Charged?

[Evil Juggalo]

Ok, so I work for Radioshack and I have 2 phones on employee plans..
Have a Sprint and an AT&T.. Currently have my AT&T line (ROKR E6) Forwarding calls to my Sprint (HTC Touch)..

Different friends use different services, but id like to know which service I lose minutes on, and what service my friends lose minutes on when doing the call forwarding.. Since my AT&T line is forwarded, do my friends on AT&T get the call for free since they called my AT&T phone that is forwarded to Sprint? Billed to AT&T or Spint? If a friend on Sprint calls the AT&T # is it a free call since its forwarded to my Sprint phone?

Help would be much appreciated..