Only the phone of Android system support Reply with message.
After the band is bound with Mi Fitness APP, open [Mi Fitness] APP >> [Device] >> [Incoming calls] >> enable [Incoming calls] and [Reply with message]. When the band is connected to the APP, you can reply to the incoming call by SMS in the band interface. Also, you can set the content of the message through the following path: [Mi Fitness] APP >> [Device] >> [Incoming calls] >> [Reply with message].
Note:
As the iOS system can't get permission to reply to SMS, it doesn't support this function.