Sheikh Muhammad b. Sâlih al-`Uthaymîn
It is permissible for a person observing i`tikâf – a retreat in the mosque – to make and receive phone calls when necessary to fulfill the needs of the Muslims.
This is the case if the phone is present in the mosque in which the worshipper is offering a retreat, so that he does not have to leave the mosque in order to make or receive the call. He may not leave the mosque in order to use the phone.
If he is a person who has specific duties towards others that prevents him from remaining in the mosque, then that person should not observe i`tikâf in the first place.
This is because fulfilling the needs of the Muslims is more important than observing a retreat in the mosque. The benefit of helping others is a benefit that extends beyond the individual. By contrast, the benefit of observing i`tikâf is wholly personal.
As a general rule, that which provides benefit to others is superior to that which only provides a benefit to the self. The only exception to this is where the act that provides individual benefit is one of the individual’s religious obligations.
And Allah knows best.
Source: Islam Today