While it is recommended to use a qualified telephone technician for installation, troubleshooting and/or repairs, you may follow the steps listed below to troubleshoot on your own.
Please note the cautions listed HERE.

Problem with one residential phone:

If only one of your phones does not work, unplug it and plug in a working phone. If you have only one phone, borrow a working phone and plug it into the jack of the non-working phone. If that phone works, you know the problem is with the original phone. Check your warranty or contact the store where you purchased the phone.

Problems with more than one residential phone:

Contact your local cable service provider.

Note: Sometimes during or after a storm you may experience no dial tone when cordless phones or answering machines are in use. Unplug your equipment for 10 to 15 minutes then reconnect it. If you still do not have a dialtone, contact your local cable service provider for service and repair.