Hello & Welcome to Just Answer,
I'm Greg and will do my best to assist you today.
Sorry to hear of any issue you are having with your phones.
To help you with this issue, may I ask:
1) The FULL Model Number of the phone system you are working with? If this is a cordless system, that model should be printed on a sticker on the bottom or back of the MAIN base.
2) What leads you to believe that your ringer is muted? For example, are there any icons or text on a display that says the ringer is not sounding?
3) What phone service provider (phone company) you have for this phone line?
4) If you have ANY other devices (phones, fax, alarm system, etc.) attached to this same phone line and if so, are they working properly?