Check the battery level of the handset. Low battery level would lead to noise. Place the handset on the charging cradle for re-charging, or replace with new batteries if necessary.
Unplug and re-plug ALL phone line cables (including those to the phone and those to other connected equipment such as a FAX machine) for several times.
Poor sound quality, or cracking noise, may be caused by a poor connection between the phone line connector and the wall connection, or between other line connections. By unplugging and re-plugging all the cables, a good connection is re-established.
Low frequency noise may be generated due to reflection of radio frequency signals to the phone from metallic surfaces (including tables made of metal or tin; and the metal casing of some electronic equipment). Relocate the base of your phone to a non-metallic surface.
In case the cracking noise persists, the problem may be due to the phone itself, or the phone line.
To find out whether it is a problem with the phone, try replacing it temporarily by another phone. If the cracking noise is still present with the new phone, it is caused by the phone line. If the cracking noise disappears, verify one more time by connecting your DECT phone again.
Cracking noise or echo may also be due to interference from DSL (digital subscriber line) connection. In this case, contact your service provider for installing a suitable DSL filter.