See our blog post on how to activate RedMi/Xiaomi’s Caller ID: "Tap here to see how to enable for MIUi6 and Mi3" for MIUI3 and MIUI6. Read below for the other MIUI's.
1. Go to Security > Auto start > Enable Truecaller
2. Go to Security > Permissions > Permissions > Truecaller > Display pop-up window > Accept
3. Go to Notifications > Manage Notifications > Truecaller > Show Floating Notifications
For Mi4i: Phone Settings > Additional Settings > Battery > Manage apps battery usage > Choose apps > Truecaller > Custom and in Background Network , choose stay connected.
Alternative solution, Phone settings > security > permissions > truecaller > allow all permissions. After that, go to additional settings > battery > manage apps battery usuage, by default its "standard" make sure to keep it off,
Battery settings > Manage background data