Commentary on the verse {That Day, everyone will follow [the call of] the Caller [with] no deviation therefrom} [Taa-Haa 20:108]
Question

Who is the Caller mentioned in the verse {That Day, everyone will follow [the call of] the Caller [with] no deviation therefrom} [Taa-Haa 20:108]?
Praise be to Allah.Firstly:
Allah, may He be exalted, says (interpretation of the meaning):
{And they ask you about the mountains, so say, “My Lord will blow them away with a blast,
And He will leave the earth a level plain.
You will not see therein a depression or an elevation.”
That Day, everyone will follow [the call of] the Caller [with] no deviation therefrom, and [all] voices will be stilled before the Most Merciful, so you will not hear except a whisper [of footsteps]} [Taa-Haa 20:105-108].
Shaykh as-Sa ‘di said:
Here Allah, may He be exalted, tells us of the horrors of the Day of Resurrection and what will occur then of earthquakes and upheaval.
{And they ask you about the mountains} that is, what will happen to them on the Day of Resurrection, and will they remain as they are or not?
{so say, “My Lord will blow them away with a blast} that is, He will remove them from their places and they will become like carded wool, or like sand. Then He will crush them and make them scattered dust, so they will diminish and vanish. He will make them level with the earth, and He will make the earth a smooth levelled plain, which will be so flat that the onlooker will see no depression or elevation, no valleys or high ground. The earth will be made smooth and flat, and will be expanded to accommodate all creatures. Allah will spread it out like a leather mat, so they will all be standing in one place, they will all be able to hear the caller and they will all be seen at one time. Hence Allah says:
{That Day, everyone will follow [the call of] the Caller} that is when they are resurrected and rise from their graves. The caller will call them to attend and gather in the gathering place. So they will follow him, hastening towards him, not turning away from him and not deviating to the right or left.
{[with] no deviation therefrom} that is, they will not deviate from his call;  rather the caller will call all the people and make them all hear, so they will all come to the place of standing with their voices stilled before the Most Gracious.
{so you will not hear except a whisper [of footsteps]} that is, the sound of their footsteps or the sound of whispering which is moving the lips only. They will be overwhelmed by fear, quiet and attentive, awaiting the judgement of the Most Gracious, utterly humbled. In that great place of standing, you will see rich and poor, men and women, free and slave, kings and commoners, silent and attentive, their gazes lowered, their necks bent in submission, kneeling in humility, not knowing what the fate of each will be or what will be done to him. Each person will be preoccupied with himself and his own situation, and he will be heedless of his father, brother, friends and loved ones. { Each one of them, that Day, will have enough concern (of his own) to make him indifferent to the others} [‘Abasa 80:37]. At that time the Just Judge will pass judgement on them and will requite the doer of good for his good deeds and the doer of evil by depriving him of reward.
The hope that we have in the generous Lord, the Most Gracious, Most Merciful, is that He will show people grace, kindness, pardon and forgiveness such as cannot be described or imagined. At that time all people will hope for His mercy because of what they see, but those who believed in Him and His Messengers will be singled out for mercy.
If it is asked: on what basis do you have this hope? Or if you wish, you may say: how do you know what is mentioned above?
We say: it is because of what we know about His mercy prevailing over His wrath, and what we know of His vast generosity which encompasses all of creation, and because of what we see in ourselves and others of abundant blessings in this realm, and especially what we know about His mercy on the Day of Resurrection."(Tafseer as-Sa‘di  p. 513).
Secondly:
Allah, may He be exalted, says: {That Day, everyone will follow [the call of] the Caller [with] no deviation therefrom, and [all] voices will be stilled before the Most Merciful, so you will not hear except a whisper [of footsteps]} [Taa-Haa 20:108],
Allah, may He be exalted, tells us about the Day of Resurrection in many soorahs, including Soorat Taa-Haa, in which He explains that on that Day, the people will follow the voice of the caller, who will call them to the place of standing, {[with] no deviation therefrom}, meaning that they cannot drift away from it; rather they will quickly move towards him.
At-Tabari said: Allah, may He be exalted, says: on that Day, the people will follow the voice of the caller of Allah, who will call them to the place of standing on the Day of Resurrection, so as to gather them in that place, {[with] no deviation therefrom} [Taa-Haa 20:108]. In other words, they cannot drift away or deviate from it; rather they will move quickly in his direction.
It was said that what is meant by {[with] no deviation therefrom} is that they cannot drift away from it, because what is meant is what we have mentioned, that they will not overtake him or drift away from him; rather they will follow him and come to him.
{and [all] voices will be stilled before the Most Merciful} – here Allah, may He be exalted, tells us that the voices of all creatures will fall silent before the Most Merciful, so He describes the voices as being stilled.
What is meant is that they will all humble themselves before their Lord, so you will not hear a word uttered by anyone who is able to speak, except one to whom the Most Merciful grants permission."(Tafseer at-Tabari  16/167).
Some of the scholars stated that the caller who will call them to the place of standing on the Day of Resurrection is Israfeel (peace be upon him). See: al-Kashf ‘an Tafseer al-Qur’an (18/60).
This was also stated by al-Balansi in his book, Tafseer Mubhamaat al-Qur’an (2/210).
At-Taahir Ibn ‘Ashoor mentioned a difference of scholarly opinion concerning that, as he said: Regarding the caller, it was said that he is the angel Israafeel (peace be upon him), who will call them with a call of subjugation and formation, so that their bodies will be restored and their souls will return to them, and they will walk with humility to the place to which they are summoned. And it was said that the caller is the Messenger, so each people will follow their Messenger. "(At-Tahreer wa’t-Tanweer  16/309).
The view that the caller is Israafeel (peace be upon him) was also narrated from Yahya ibn Salaam and Muqaatil ibn Sulaymaan.
See: Mawsoo‘ah at-Tafseer al-Ma’thoor (14/412).
And Allah knows best.