Hi,
What indication do you get when you call the number of the phone from another phone?
Have you set the phone to "silent" or mute"?
Have you checked the ringtone Volume level?
Can you make an o/g call OK?
Have you checked that you are not in airplane mode?
Have you proved that the loudspeaker works by playing music? (This speaker is the one used for the ringtone)
Have you checked that you have selected a ringtone?
Switch off/one once or twice.
Also try go to Settings > Device > Sound > Phone Ringtone and select a different ringtone, do you hear the ringtone there?
Or finally It could be a software/App issue and a hard/factory reset may be what is needed. You can select your ZTE Phone model from the link below for instruction to hard reset it. Hard Reset will erase all of your data, back up info. prior unless it does not matter. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.