Is it permissible to interrupt an obligatory prayer upon hearing the doorbell or a telephone ringing? How should one leave the prayer?

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.

When a person starts an obligatory prayer, it is prohibited to interrupt it unless there is a necessity such as guarding someone from damage or harm, protecting wealth from being lost, and so on. However, answering a phone or opening the door do not fall under these necessities. One can move a few steps to open a door provided that he does not turn away from the Qiblah (i.e. direction of prayer).

As for how to interrupt a prayer, when it is permissible, you can do this with Tasleem (i.e. saying: As-Salaamu ‘Alaykum) or by simply moving away.