The phone has a multi-beep tone when the battery is fully charged. There is an older thread on the topic re: the Gusto 2.
According to the manual, there should be an entry in the sound settings, alert sounds:
Battery Charge Alert: Choose Alert or Vibrate to have the phone
alert you when battery charging is complete, or choose Off.
However, that menu pick is not actually available on the phone. The last entry in that menu is “Device Connect,” which in the manual, is the item right above the entry that appears to be missing.
Brand new phone – provisioned Friday of last week. S/W V, B311V.N.