The iqaamah (call immediately preceding the prayer) has been proven in several versions from the Prophet (blessings and peace of Allah be upon him), such as the following: 

Saying each phrase once, apart from the takbeer (“Allaahu akbar”) at the beginning and the end, which is said twice, and the phrase Qad qaamat is-salaah (prayer is about to begin), which is also said twice. So the number of phrases is eleven in total. This is the madhhab of the Shaafa‘is and Hanbalis, and is also the opinion of the Maalikis, but they also say Qad qaamat is-salaah (prayer is about to begin) only once. 

See: al-Mughni (2/59); al-Mudawwanah (1/179). 

This is the iqaamah of Bilaal, the mu’adhdhin of the Messenger (blessings and peace of Allah be upon him). 

According to the hadeeth of ‘Abdullah ibn Zayd about the adhaan:

He said: Then he went a short distance away from me, and said, “When the prayer is about to start, say, Allaahu akbar Allaah akbar, ash-hadu an laa ilaaha ill-Allaah, ash-hadu anna Muhammadan Rasool-Allaah. Hayya ‘ala’l-salaah, hayya ‘ala’l-falaah. Qad qaamat il-salaah, qad qaamat il-salaah. Allaahu akbar, Allaahu akbar, laa ilaha ill-Allaah (Allaah is most Great, Allaah is most Great. I bear witness that there is no god but Allaah. I bear witness that Muhammad is the Messenger of Allaah. Come to prayer, come to success. The prayer is about to begin, the prayer is about to begin. Allaah is most Great, Allaah is most Great. There is no god but Allaah).” When morning came, I went to the Messenger of Allaah (peace and blessings of Allaah be upon him) and told him what I had seen. He said, “This is a true dream, in sha Allaah.”

Narrated by Abu Dawood (499). Al-Albaani said: (It is) hasan saheeh. 

It was narrated that Anas ibn Maalik (may Allah be pleased with him) said: Bilaal was ordered to say the phrases of the adhaan twice and the phrases of the iqaamah once.

Narrated by al-Bukhaari (605) and Muslim (378). 

It was narrated that Ibn ‘Umar (may Allah be pleased with him) said: In the adhaan at the time of Messenger of Allah (blessings and peace of Allah be upon him) each phrase was said twice, and in the iqaamah each phrase was said once, except that he would say: Qad qaamat is-salaah, qad qaamat is-salaah (prayer is about to begin, prayer is about to begin).

Narrated by Abu Dawood (510); classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

In another version, the phrases of the iqaamah are exactly like the phrases of the adhaan, with the addition of the phrase Qad qaamat is-salaah (prayer is about to begin), which is said twice. So the number of phrases is seventeen in all. This is the view of the Hanafis and some of the Shaafa‘is. 

See: al-Mabsoot (1/219). 

This is the iqaamah of Abu Mahdhoorah (may Allah be pleased with him), which was taught to him by the Messenger of Allah (blessings and peace of Allah be upon him). 

It was narrated that Abu Mahdhoorah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) taught me the iqaamah with seventeen phrases: “Allaahu akbar, Allaah akbar, Allaahu akbar, Allaah akbar; ash-hadu an laa ilaah ill-Allaah, ash-hadu an laa ilaah ill-Allaah; ash-hadu anna Muhammadan rasool-Allaah, ash-hadu anna Muhammadan rasool-Allaah; hayya ‘ala as-salaah, hayya ‘ala as-salaah; hayya ‘ala’l-falaah, hayya ‘ala’l-falaah; qad qaamat is-salaah, qad qaamat is-salaah;  Allaahu akbar, Allaahu akbar; Laa ilaaha ill-Allaah. 

Narrated by Abu Dawood (502) and at-Tirmidhi (192); classed as saheeh by al-Albaani. 

Everything that is proven from the Prophet (blessings and peace of Allah be upon him) is Sunnah and should be followed. So there is nothing wrong with the mu’adhdhin reciting the iqaamah of Bilaal or the iqaamah of Abu Mahdhoor (may Allah be pleased with them both), although what is better is to do one sometimes and the other sometimes, so that the entire Sunnah will have been followed. 

Shaykh al-Islam Ibn Taymiyah said, after noting that the hadeeths indicate that saying the iqaamah in both forms is permissible: 

As that is the case, the correct view is the view of ahl al-hadeeth and those who agreed with them, which is to accept everything that has been proven concerning that from the Prophet (blessings and peace of Allah be upon him) and not to reject any of it, because varying the format of the adhaan and iqaamah is like varying the format of recitations and tashahhuds and the like. No one has the right to reject any precedent or Sunnah that the Messenger of Allah (blessings and peace of Allah be upon him) established for his ummah. As for those who have gone so far as to disagree and split to the point of forming friendships, feeling enmity and fighting on the basis of such matters, in which either of the two versions are acceptable, they come under the heading of those who divide their religion and break up into sects (cf. 6:159). To follow the Sunnah properly in such cases means doing it one way sometimes and the other way sometimes, one way in some place and another way in another place, because abandoning what has been narrated in the Sunnah and adhering to something else may lead to what is Sunnah being regarded as an innovation and what is mustahabb being regarded as obligatory, which would lead to division and differences if others follow the other way. The Muslim should pay attention to general principles which include adhering to the Sunnah and the main body of Muslims (as-sunnah wa’l-jamaa‘ah), especially with regard to matters such as prayer in congregation.

End quote from Majmoo‘ al-Fataawa, 21/66 

He also said: 

No one has the right to take the words of one of the scholars as a slogan that must be followed and forbid anything else that is mentioned in the Sunnah; rather everything that is mentioned in the Sunnah is broad in scope, such as the adhaan and iqaamah. 

The one who says the phrases of the iqaamah twice has done well and the one who says them once has done well, but the one who regards the one way as being obligatory but not the other is mistaken and has gone astray. And the one who regards as a friend the one who does it one way but not the other just because of that is also mistaken and has gone astray.

End quote from Majmoo‘ al-Fataawa, 22/46 

And Allah knows best.