idivert is specifically designed to work with Voicemail and uses the Voicemail Profile and thus the Voicemail Pilot Number for diversion.
For a "one up" type of setup you might be able to create a new Voicemail Profile and fake the Voicemail Pilot number but I wouldn't even go down that path unless I absolutely had to.
Have a look;
The Immediate Divert feature allows you to immediately divert a call to a voice-messaging system. When the call gets diverted, the line becomes available to make or receive new calls.
Access the Immediate Divert feature by using the iDivert softkey. Configure this softkey by using the Softkey Template Configuration window of Cisco CallManager Administration. The softkey template gets assigned to phones that are in the Cisco CallManager system.
The call that is being diverted can be in the call offering, call on hold, or call active state. The call can be incoming or outgoing. The person on the call that is being diverted will receive the greeting of the voice-messaging system of the person who diverted the call.
A voice-messaging profile can represent either a specified voice-messaging profile or a default voice-messaging profile. (Choose default voice-messaging profiles by choosing None in the Voice Messaging Profile drop-down list box in the Directory Number Configuration window.)
A voice-messaging pilot in the voice-messaging profile identifies the voice-messaging system to which redirected calls go. The combination of a directory number and voice-messaging mask defines the voice-messaging mail box.
If this functionality is for all of your users this won't be too helpful, but if this is for some "Manager type" users you can use IPMA. In IPMA there is the ability to specify a Target number for idivert which is not the Voicemail number. Have a look;
How to Configure the Divert Target for a Manager
Managers using Cisco Unified CallManager Assistant in proxy-line mode can use the Divert All (DivAll) and Immediate Divert (ImmDiv) features to send calls to the assistant or to another phone number (known as a target). The DivAll feature and the ImmDiv feature share the same Divert target.
Managers using Cisco Unified CallManager Assistant in shared-line mode can set up a Divert target and forward calls as the calls come in by using the ImmDiv softkey. The divert screen automatically displays when you log in.
By initial default, the Divert target is the manager's active assistant. Managers and assistants can use the procedure below to change this target.
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