Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
I- If the doorbell rings while you are offering a voluntary prayer, there is nothing wrong with stopping your prayer and seeing who is at the door.
2- If it is an obligatory prayer, then you should not rush your prayer unless it is something very important that you fear will be missed.
3- If possible, you may alert the person at the door that you are busy in prayer, by saying subhanallah for men or clapping for women.
Answering your question, the late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
If the prayer is a voluntary prayer, the matter is much easier as there is nothing wrong with stopping your prayer and seeing who is at the door.
However, if it is an obligatory prayer, then you should not rush your prayer unless it is something very important that you fear will be missed.
If possible, you may alert the person at the door that you are busy in prayer, by saying subhanallah for men or clapping for women, that would be sufficient. The Prophet (peace be upon him) said,
“If anything happens to someone during the prayer, the men should say subhanallah and the women should clap.” (Abu Dawud)
However, if that will not help because the door is far away and the person will not hear you, then there is no harm in you breaking off the prayer, in particular if it is a voluntary prayer. If it is an obligatory prayer, you should only do that if it is expected that that is something very important.
In that case, you may break off the prayer and then repeat it from its beginning afterwards.
Almighty Allah knows best.
Source: Islamic Fatawa Regarding Women, Compiled by Muhammad Ibn Abdul-Aziz Al-Musnad and translated by Jamal Al-Din Zarabozo, Darussalam, 1996.