So there could be a couple things going on here...
First, I am confused how you are forwarding to an external number. If memory servers me correctly you cannot do that from the phone directly, but have to forward it to a System Speed Call entry that goes to the external number.
There is also a couple system options that to be enabled, but since you did this before those are likely fine.
Is the cell number you are trying to forward to a local call? One issue we see when a call is forwarded off-site, is the original callers caller ID information is presented to your carrier as the "originating number" and not your company's number, so many carriers will immediately reject the call. The common symptom here is you can call the extension from another internal phone and it work, but when you call the DID from an external phone it fails.