Praise be to Allah.
If the worshipper is in the position that you describe and the phone starts ringing, it is permissible for him to lift the receiver, even if he has to move a little way forward or back, or to the right or left, on the condition that he keeps facing the qiblah and he says ‘Subhaan Allaah’ so that the person who is calling will know (that he is praying). It was narrated in al-Saheehayn that the Messenger of Allaah (peace and blessings of Allaah be upon him) used to pray whilst carrying his granddaughter Umaamah. When he did rukoo’, he would put her down, and when he stood up he would pick her up again. According to a report in Muslim, this was when he was leading the people in prayer in the mosque. (Narrated by Maalik, 1/170; al-Bukhaari, 1/487, in Sutrat al-Musalli, Baab idha hamala jaariyatan sagheeratan ‘ala ‘unuqihi; Muslim, 543, in al-Masaajid, Baab jawaaz Hamal al-Subyaan; Abu Dawood, no. 917-920; al-Nasaa’i, 2/45).
Ahmad and others narrated that ‘Aa’ishah (may Allaah be pleased with her) said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) was praying in his house with the door closed. I came (to the door) and he walked over and opened the door for me, then he went back to his place.” She mentioned that the door was in the direction of the Qiblah. (Narrated by Ahmad, 6/31; Abu Dawood, 922; al-Nasaa’i, 1/178; al-Tirmidhi, 2/497).
Al-Bukhaari and Muslim narrated that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever notices something alarming whilst praying, then men should say ‘Subhaan Allaah’ and women should clap their hands.”
And Allaah is the Source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.