Check your call forwarding settings:
- If you are using 4G (LTE), switch the Highest connection speed down to 3G in the cellular settings, then you will see the options for call forwarding.
- Depending on your phone, select Settings > network+ > set, or tap Phone > … > settings.
- Switch Call forwarding to On.
- Check that the number to which you want to forward your calls and other forwarding options are correct.