From Monday 13 August 2018, only GSPS handsets with the latest firmware version will be able to operate within India. Those handsets running on previous firmware versions with SIM cards issued by partners other than BSNL  will be automatically rejected by the Inmarsat  network when placing calls originating inside India.

IsatPhone Pro support page

IsatPhone 2 support page

Inmarsat GSPS handsets like ISATPhone2 or Pro need Firmware update to be used in India