Assalamualaikum ustaz. What is the ruling if the bilal straight away calls the iqamah after the adhan because at the time there are no one who is performing any sunnah prayer before the congregational prayer?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
The adhan is one of the eminences of Islam with the purpose of informing people that the time for the obligatory prayer has begun. Likewise, the iqamah is called to inform others that the prayer will be performed. The adhan is sanctioned as soon as the time for prayer begins as stated in a hadith narrated by Malik bin al-Huwairith RA, he said that Rasullullah PBUH said:
فَإِذَا حَضَرَتِ الصَّلاَةُ فَلْيُؤَذِّنْ لَكُمْ أَحَدُكُمْ
“And when the time for prayer comes one of you should call the adhan,” 
In madhhab Imam al-Syafi’e, the ruling of iqamah is sunnah muakkad for the five obligatory prayers the same as the ruling for the adhan. Imam al-Nawawi said: What’s well known in our madhhab is that both of them (adhan and iqamah) are sunnah and all who are present (not travelling) and also travelling (prayer) for congregational and alone prayer. 
This opinion is also supported by several jurist scholars such as madhhab Maliki. This is as stated by Imam Ibn Rishd regarding the ruling for iqamah: Indeed it (the iqamah) according to jurists is the right for those who pray alone and in congregation it is sunnah muakkad and according to the zahiriah it is obligatory. According to the first opinion, the prayer isn’t invalid when it is neglected (the iqamah), while according to the second opinion, it is invalid. 
Moreover, according to the jurists, it is sunnah to separate between the time of adhan and iqamah with a certain duration of time so that people will have the time to prepare for prayer. This follows the hadith narrated by Jabir bin Abdullah, where the Prophet PBUH said to Bilal:
وَاجْعَلْ بَيْنَ أَذَانِكَ وَإِقَامَتِكَ قَدْرَ مَا يَفْرُغُ الآكِلُ مِنْ أَكْلِهِ وَالشَّارِبُ مِنْ شُرْبِهِ وَالْمُعْتَصِرُ إِذَا دَخَلَ لِقَضَاءِ حَاجَتِهِ وَلاَ تَقُومُوا حَتَّى تَرَوْنِي
“And leave between your adhan and your iqama time for one who eats to finish his food and one who drinks to finish his drink, and one who needs to relieve himself to do so. And do not get up to pray* till you see me do so.” 
Al-Khatib al-Syirbini stated: “It is sunnah for the muezzin and imam to separate between the time of adhan and iqamah with an appropriate time for people to gather and perform prayer and with the suitable time for sunnah prayer before they perform the obligatory prayer.” 
Coming back to the above question, it is permissible for the bilal to immediately call the iqamah. However, it is prioritized for the bilal himself to perform the encouraged qabliah sunnah prayer or give some time. This is so because Abdullah bin Mughaffal al-Muzani RA narrated that Rasullullah PBUH said:
بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ، بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ، ثُمَّ قَالَ فِي الثَّالِثَةِ: لِمَنْ شَاءَ
“Between every pair of adhans, there is a prayer; between every pair of adhans there is a prayer.” Then he said on repeating it a third time, “for him who wishes.”