Is It Permissible to Call People for the Janazah Prayer?
Question

Is it prescribed to call the people via loudspeakers by saying “as-salatu jami`ah (prayer is about to begin)”?.
Praise be to Allah.It is not prescribed to give the call via loudspeakers for worshippers to assemble for the funeral prayer, whether that is by saying “As-Salatu Jami`ah (prayer is about to begin)” or saying “As-Salatu `ala Al-Mayyit (prayer for the deceased)”.
Al-Bahuti (may Allah have mercy on him) said: No call should be given for the funeral prayer or Tarawih, because that is an innovation. Even worse than that is what is done at the funeral prayer of reciting poetry or mentioning characteristics, most of which may be lies. In fact that comes under the heading of wailing for the dead. (End quote from Kashshaf Al-Qina`, 1/234)
Imam Ash-Shafi`i (may Allah have mercy on him) said: “… With regard to the funeral prayer and all optional  prayers apart from `Eid prayer and the eclipse prayer, there is no Adhan (call to prayer) for them or call by saying “as-salatu jami`ah (prayer is about to begin)”. (End quote from Sharh Al-Muhadhdhab, 3/83)
In Al-Mawsu`ah Al-Fiqhiyyah (16/7) it says: In Ash-Sharh As-Saghir it says: It is disliked to shout in the mosque or at its door, saying, “So and so has died, hasten to his funeral!” However, announcing it in a low voice without shouting is not disliked. (End quote)
As-San`ani (may Allah have mercy on him) said: His saying in Ash-Sharh, that it is recommended when calling people to prayer on `Eid and other occasions when it is not prescribed to give the Adhan, such as the funeral prayer, to say “As-Salatu Jami`ah (prayer is about to begin)”, is not correct, because there is no evidence to suggest that it is recommended. If it were recommended, the Prophet (blessings and peace of Allah be upon him) would not have omitted to do it, and neither would the Rightly Guided Caliphs after him. Yes, that is proven in the case of the eclipse prayer, but it is not proven in other cases and it is not valid to draw an analogy, because if the same reason existed in his time but he did not do it, doing it after his time is an innovation. So it is not correct to affirm it on the basis of analogy or otherwise. (End quote from Subul As-Salam, 1/184)
For more details, please see the following answers:
Ruling on making death announcements from mosque minarets via loudspeaker
How to Pray Salat al-Janazah
Is the Funeral Prayer Obligatory?
Can You Offer The Funeral Prayer in Absentia?
Can Women Offer Janazah Prayer?
And Allah knows best.