It is obligatory to call the adhan in mosques and wherever people meet regularly to do the prayer. It is clear from his words that there is no difference between the communal mosque, i.e. that in which Jumu'a is held, and mosques other than communal. There is also no difference between the mosques being close together or whether there is one mosque on top of another. It is also done is places where groups meet regularly to pray. It is clear that it is done whether it is in mosques or elsewhere since they seek to call others to the prayers.
Every group seeks others.
Even if it is not a regular prayer, then it is sunna for there to be an adhan. The irregular group differs from the irregular in that it is a group in a settled place where they do not wait for others in a place outside of a mosque, and so the adhan is not sunna for them nor it is recommended. Indeed, it is disliked. However in a journey, the adhan is recommended. It is recommended for someone who is alone on a journey to do the adhan.