EL50003 handset works only as a speakerphone

When dialing or on a call sound is only working for one of our handsets when it's used as a speakerphone. In other words when making a call we can't hear the dialtone from the earpiece of the handset; and when on a call we don't hear any sound from the earpiece of the handset. However dialtone and calling sounds work normally if we use the speakerphone mode of the handset. Affects one specific handset only. Hoping there is a simple way to troubleshoot the cause of this behaviour and correct the problem. If not, we'll just live with using this handset only in speakerphone mode. Checked all of the various volume settings and even paired the problem handset with a different phone main base as a test (problem handset is from my mother's landline phone system; I paired it with similar phone system in my house  ...). No joy, problem persisted.  I could make and answer calls from the problem handset but only hear dialtone or sound when in speakermode.