Do obsessive-compulsive disorder and illness, when afflicting a worshipper, justify interrupting one's prayer to answer a caller or an uninvited guest, and what is the ruling on his prayer and the prayer of those who pray behind him in such a case?

It is not permissible to interrupt the prayer to answer the phone or open the door, except to save a life or property from destruction. The mere thought of interrupting the prayer does not invalidate it as long as the worshiper resists it, and such thoughts are from the whisperings of Satan that must be ignored. If your prayer is valid, then the prayer of those who pray behind you is valid.