Yes, the SIP client will register do the Session Manager via two routes:
1 - if you are off the enterprise WIFI, then the client will register through the SBC to SM. This connection could be achieved via the data leg of the cellular phone, or via any WIFI network the cellular device is connected to. In the case of cellular data, then data will be utilised for all call signalling and voice traffic (which differs from one x mobile non sip - where the signalling component traverses data, but voice is carried via trunks)
2 - the other way is using enterprise wifi - which like any other wifi - will not incur any cellular data charges. This can also be achieved by bypassing the SBC and going straight to SM as its internal to your enterprise, but this requires network config in terms of DNS routing.
The main thing to remember is - You will only incur cellular data charges when connecting via the cellular data network. The application allows you to allow or deny this, so you can stipulate that WIFI access is the only method, thus eliminating these charges.