In the Name of Allah, Most Gracious, Most Merciful.
All thanks and praise are due to Allah and peace and blessings be upon His Messenger.
In this fatwa:
1- A person observing i`tikaf is recommended to forget about the allurements of this world and never occupy himself with it.
2- Using the cell phone should be limited and confined to one’s basic needs. Also, one can use it in fulfilling the needs of his fellow Muslim brothers.
With regard to your question, Sheikh `Attiyah Saqr, former head of Al-Azhar Fatwa Committee, states:
In principle, i`tikaf is a Sunnah. It becomes obligatory once the person vows to make it. In this sense, when a person vows to perform i`tikaf, it becomes incumbent on him to do it within a certain span, if he specifies a certain time for it, if not it can be done at any suitable time. If a person fails to do that, he is to make up for it in case of obligatory i`tikaf and in the case of Sunnah i`tikaf, making up for it is recommended.
As for using cell phone, we can say that a person observing i`tikaf is recommended to forget about the allurements of this world and never occupy himself with it. In other words, using the cell phone should be limited and confined to his basic needs. Also, he can use it in fulfilling the needs of his fellow Muslim brothers.
As it is permissible for a person to go out of the mosque at any time, using cell phone is permissible in the mosque, in the sense that as a person is allowed to get out of mosque for his personal needs, he is allowed to use cell phone as long as it is confined to his personal needs or for righteous deeds.
It is reported that when one called him to fulfill his need, Ibn `Abbas (may Allah be pleased with him) rushed to him. As one of those observing i`tikaf stopped him, he said: “I have heard the Messenger of Allah (peace and blessings be upon him) saying: ‘To answer the need of your brother is better in reward than spending ten years in i`tikaf.‘
Allah Almighty knows best.