All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
Using people`s property without their permission is an act of transgression and it is highly forbidden. The evidence on this is that Abu Bakrah (May Allah be pleased with him) said: Delivering the sermon during the Farewell Pilgrimage on the day of Sacrifice at Mina, the Messenger of Allah (PBUH) said: "Verily your blood, your property and your honour are as sacred and inviolable as the sanctity of this day of yours, in this month of yours and in this town of yours. Verily! I have conveyed this message to you." [Al-Bukhari and Muslim].
The degree of sanctity and inviolability even multiplies when there is betrayal of trust. Since an employee is entrusted with the property of his employer, it isn`t permissible for him to use that property or dispose of it without the permission of the latter, and if he does so then he is considered sinful and will be held to account for that before Almighty Allah unless the owner forgives him.
It isn`t permissible for you (Asker) to use the restaurant`s phone without the owner`s permission.
As for using your own phone while at work, this is permissible so long as it is done at a modest level and doesn`t affect work. And Allah The Almighty Knows Best.