You can have an intercom conversation with non-Sena Bluetooth headsets using the Universal Intercom feature. Non-Sena Bluetooth headsets can be connected to the Sena Bluetooth headset if they support the Bluetooth Hands-Free Profile (HFP). In order to connect to a Sena headset, the non-Sena headset needs to perform the button operation to redial or make a phone call, by using the voice dialer, in order to connect to the Sena headset for intercom communication since the non-Sena headset will act as if the Sena headset is another phone rather than a Bluetooth headset. In order for the non-Sena headset to disconnect the intercom then they need to perform the button operation to end a phone call. The Sena headset can connect and disconnect to the non-Sena headset using the same operation as connecting to another Sena headset. Finally, the working distance may differ depending on the performance of Bluetooth headset that it is connected to. In general, it is shorter than normal intercom distance since it uses the Bluetooth Hands-Free Profile.
You are not using your cellular service at all. You are using the voice dialing feature on the non-Sena headset to connect to the Sena headset.