All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
A prayer is not invalidated if the worshipper does not answer his ringing cell phone. However, it would have been better for him to switch it off with a slight movement that does not undermine the validity of the prayer (as it is done for the interest of the prayer).
It would have been better if the worshipper switched off the cell phone before joining the prayer so as not to cause any disturbance to the worshippers during the prayer.
Allaah Knows best.