For Android
1. Launch the Phone app.
2. Tap on the menu icon with 3 vertical dots then go to Settings.
3. Tap on Supplementary services.
4. Tap on Call forwarding, then Voice call.
5. Ensure that all voice call forwarding features have been disabled.
For iOS
1. Go to Settings then tap on Phone.
2. Tap on Call Forwarding.
2. Ensure that it is disabled.