ZTE ZMAX - Manual (Page 73)

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To configure your phone to receive Internet calls:
By default, your phone is configured to make Internet calls, but not to
ensure that you will receive them. But you can configure your phone to
listen for Internet calls made to an Internet calling account you added to
your phone.
1. Touch the
Home Key > > > Settings > Accounts.
2. Check Receive incoming calls. Checking this setting will reduce
your battery life between charges.