Ibn Rajab al-Hanbali stated in his Fath al-Bari (8/220-221) (arabic here):

The first adhaan for Jumu'ah is introduced, it was introduced by Uthmaan. He saw that (the adhaan) will not be heard unless he increases the mu'adhdhins so that those furthest away will be informed (of time of Jumu'ah) and hence it became a Sunnah, because it is upon the Khulafaa to look into such matters for the (benefit of) the people. And from this it is understood that this returns to the opinion of the Imaam. If he is need of it (an extra adhaan), due to the abundance of people, he can do it, and if not, then there is no need for it.

And later on page 231:

And his statement in this narration which was related by al-Bukharee here, "And so the affair became established upon that", indicates that this was when Uthmaan ordered it, it continued and it was not abandoned after that. And this shows that Alee remained upon it and did not invalidate it, for two of the rightly-guided Caliphs agreed upon its performance, may Allaah be pleased with them all.

Note: Please refer to this article to see how Yahya al-Hajuri clipped this statement of Ibn Rajab in order to defend his erroneous opinion that Uthmaan innovated into the religion (with a blameworthy innovation).

The above statement is a proof against the Hajurites in that the wisdom and reason for Uthmaan (radiyallaahu anhu) instituting the first adhaan is due to a need, and wherever that need is present the action can be followed and performed (doing a first adhaan). This is also the explanation of Imaam al-Albanee (rahimahullaah). However, the claim of the Hajurites that this situation no longer exists today cannot be used to claim that the action of Uthmaan (radiyallaahu anhu) was an innovation, since Uthmaan's action was legitimate and for a valid beneficial interest and it remains a Sunnah for all time that can be resorted to as and when the situation demands it. If for argument's sake we said there is no place on this Earth today where that situation exists (for which Uthmaan instituted the first adhaan) that would still not make the action of Uthmaan to be a blameworthy bid'ah, as is clear. So there are a number of crimes of al-Hajuri (and his followers). First al-Hajuri clipped the speech of Ibn Rajab and did not complete it. Second, when you read the full paragraph as occurs above, it is clear that Ibn Rajab says that the action became a Sunnah (al-Hajuri ommitted this) and that it is implemented when the need arises.

From this it becomes clearer and clearer that the Hajurites are upon falsehood, and to defend their falsehood they are relying upon the errant opinion of Shaykh Muqbil (rahimahullaah). Please refer also to this article on the sayings of the Scholars regarding the narration of Ibn Umar about the first adhaan of Uthmaan (radiyallaahu anhu).