In the masjid that I attend the sisters are unable to see the imaam during congregational salat and an intercom system is required for them to hear the imaam. Sometimes the intercom system fails during the salat leaving the sisters unable to follow the imaam unless someone watch the imaam from the window (that divides us from the men) and makes takbeer for us, or the sisters continue the salat individually. What is the correct way for us to complete our salat when the intercom fails?
If the sisters cannot see the imaam or anyone who is behind him, and the intercom system fails, then each individual should make the intention (niyyah) to pray alone independently of the imaam, and should complete the remainder of the prayer on her own. If there is someone who can see the imaam or the people praying behind him, or can hear the imaam’s voice, and can raise his voice and convey the imaam’s words (takbeer) to the others, then it is OK to continue following the imaam. And Allaah knows best.