If you want a user without a device then you have to do some special programming.
1) You need to program an extension for him. This can be a hunt group with no members or a "phantom phone" or a key on any phone including a phantom phone.
2) create a system speed dial code to route to users cell phone. Be sure to check "override toll control".
3) In call rerouting always form create a new instance with the target of the new speed dial you just created. Do not use '1'.
4) In call rerouting assignment form look up the new phantom extension you created for him in step 1. Apply the Call Rerouting Always alternative to the day, N1 & N2 columns. In the 1st alt column assign the instance that points to VM on RNA - usually 2.
5) test.
Ralph