The AT&T Microcell was a "network extender" femtocell device manufactured by Cisco and sold by AT&T between 2009 and 2017.
MIcrocells utilized a home's broadband Internet connection to create a small 3G cellular network, allowing users to make calls even if cell service was previously weak. Essentially the unit operated like a "mini cell tower" in your home.
AT&T discontinued the MicroCell in 2017, but existing devices will continue to work until AT&T's 3G network is shut down in February 2022.
If you have a phone that supports WiFi Calling and reliable broadband Internet, then you can potentially replace your Microcell with WiFi Calling. However, for users with unreliable broadband, a signal booster may be a better choice.