Chief Rabbi Yitzchak Yosef paskened today that everyone should leave their cellphones turned on over Shabbos - even those who have not been checked and tested for CoronaVirus and are not waiting for results.

The reason every person should leave the phone on is to be available in case it is discovered someone he or she was in close contact with tested positive for COVID-19 and he has to be contacted to go into quarantine because of that exposure. The Health Ministry representatives would need be able to reach such people to inform them of the need for quarantine.

As well, cars should be sent around the religious and Haredi neighborhoods with bullhorns announcing quarantine requirements, should it be necessary.

In addition, the shuls must all be shut and locked and people should daven at home, both on Shabbos and during the week. The shul has become a dangerous place of exposure and there are people more susceptible and more in danger and at risk.

Rav Dovid Yosef, rav of Har Nof and member of the Council of Sages of Shas, has also called for the shuls to be shut down and for people to daven at home. Many other rabbis have said the same, while some say if a shul can work out a minyan with complete adherence to Ministry of Health guidelines, such as only 10 people with each person no closer than 2 meters form the other, it is ok to have such a minyan.
source: Haredim10, Kikar

Regarding the cellphone, I am not sure why such a psak is needed. I was not aware of an obligation to shut off one's phone for Shabbos. I leave mine on but set it on silent mode. The psak should be more specifically about answering the phone should it ring, as Rav Yitzchak Zilbershtein paskened yesterday