There is absolutely nothing wrong with calling the athaan in the musallah; on the contrary, it is better than abandoning it altogether. Prophet Muhammad (peace be upon him) taught two men who were traveling how to make the call for prayer while traveling, where no mosques or musallahs may be found. As was narrated in the hadeeth saheah by Malik Ibn Al-Huwayrith: "My cousin and I came to the Prophet SAWS and he said to us, ‘If you travel, (meaning while you are traveling) perform the call for prayer, and call the iqamah before prayer and let the older one of you be the Imam." Narrated by Al Bukhari, Fath ul-Baari hadeeth #658 and Al Tirmidhi, hadeeth #189. According to Abu Eissa (Al Tirmidhi) this hadeeth is hasan saheeh. There were two views on the hadeeth. The majority of the Muslim scholars agreed on the validity of calling the aathaan while traveling. Others however, have said that the iqamah alone is sufficient, and that the purpose of the aathaan is to gather people for prayer. According to Ahmad and Ishaaq, the first view is more sound.
In another Hadeeth narrated by ‘Ukbah bin ‘Aamir he said: I heard the Prophet say, "Allaah is pleased by a shepherd who, while on the top of a mountain, calls for prayer and prays. Allaah the Almighty says, ‘Look at my worshipper—he calls the aathaan, and he prays; he fears me. I have forgiven my worshipper, and I have sent him to heaven ( meaning that Allah chose for this person to go to heaven).’" Narrated by Al-Nisaa’i, Abu Dawood, and Ahmad and was confirmed sound by Al Albany in his book Saheeh Al-Jami’, Hadeeth #8102.
The benefits of the aathaan are many and they are by no means limited to gathering people, or announcing prayer time. They encompass: repelling Satan, getting rewarded in the afterlife for calling for prayer, and furthermore, for announcing and repeating the oneness of Allaah, etc.
Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid