All calls are via the office?
The land line has gone faulty. I'll call for help on my mobile...
TeleWare has launched an enterprise MVNO that routes every call through the office PBX, reducing the mobile carrier to a bit pipe at best. The new offering – dubbed TeleWare Mobile, with all the panache typical of enterprise offerings – intercepts mobile calls at the operator to route them into the office systems, where all …
I don't see that as a technical drawback. You're probably already using landlines, VOIP lines and mobile connections from the PBX for least cost routing anyway. So if one line drops, the PBX will simply route calls via another connection. The calls might be a bit more expensive, but nobody aside from an admin even needs to notice that the landline failed.
There will be some privacy issues, particularly when employees use their business phones for personal calls (as quite a few of us do, because we hate to carry two different mobiles). If you're logging and/or recording all calls, that might be the bigger objection to this.