This is a fatwa from Sheikh ibn Baz (رحمه الله) that was translated into English. He was asked:
"If I was offering prayer and the door bell rang and there was no one at home but me, what shall I do? If I break my prayer, will I be sinful?"
Then ibn Baz (رحمه الله) answered:
"That is to know who is knocking, but if you were offering an obligatory prayer, it is not permissible to break it unless you are afraid to miss something important. Men could say Subhanallah, and women could clap to make the people knocking know that they are engaged in Salah. That would be enough and there is no need to break Salah. The Prophet (ﷺ) said: 'If something unusual happens during Salah, men should say Subhanallah, and women should clap.' (1) So, if it is possible to make those knocking understand that one is engaged in prayer by clapping (for women) and saying Subhanallah (for men), that would be enough instead of breaking the prayer. But if it is not possible to do so as a matter of distance or they could not hear, there is no blame in breaking the prayer if it is a supererogatory prayer. For obligatory prayers, it is only permissible to break them if one fears to miss something important or urgent and then wants to resume prayer from the beginning. All Praise is due to Allah." (2)
May Allah bless you, and shukran.
1. Agreed upon by al-Bukhari and Muslim.
2. Majmoo al-Fatwa of ibn Baz 11/109.