Isaiah is in the temple to worship YHWH.
Before Isaiah ever had this experience, he had a pattern of coming to worship.
Isaiah and the nation were in a time of grief and loss – Uzziah had been king for 52 years and Isaiah’s experience is in the year of Uzziah’s death (though we don’t know if Uzziah has yet died, if not he is likely ill and there is thus some air of uncertainty, “Will the next king serve God and bring blessings on the nation?”
Encounter with God brings humility, awe, fear, sense of one’s own sinfulness and unworthiness.
Encounter with God is visual, auditory, olfactory.

Isaiah was not looking for a call or encounter of this kind – he was simply there to bring his offering. We hope for and look for encounter with God, but that encounter comes in God’s time and way, not our own.

The actual call comes only after Isaiah’s experience of humility, confession (personal and national), repentance, and cleansing – an experience of preparation not unlike the experience of being born again and baptized – a physical symbol of cleansing akin to the coal touching Isaiah’s lips.