As a visitor of quite a few months standing in Jerusalem and visiting the Old City every Saturday to pray is it permitted due to the current situation to carry a cellular phone (switched off) in case of an emergency?

Only the police, the army or other authorized security professionals like the GSS determine what are necessary or recommended security precautions that would allow carrying muktze on shabbat. As long as there have been no instructions recomending carrying cell-phones on shabbat in Jerusalem, they should not be carried.