Replace Ericofon Gasket
You can buy the gasket here

Unscrew the Ericofon base. There are 4 phillips head screws

Remove the ericonfon base and pull it away from the body

Pull the gasket off from the base of the Ericofon phone
Put the new gasket around the base of the ericofon

Screw the four screws back on the the Ericofon body. DO NOT OVERTIGHTEN THE SCREWS!
If you screw the screws too tight you will crack the plastic base.