Hollow Sound
From BGNV Wiki
This is generally due to a high level of echo with a small amount of delay. For example, if an IP phone was connected over a LAN to a VoIP Gateway then the delay would be very small – echo that occurred on the trunk side of the Gateway may cause “hollowness” in the IP Phone.
[edit] Impact
Users may report that calls sound "hollow", "cavelike" or "tunnel-like".
[edit] Resolution
Initially try and locate the source of echo - often on the analog loop connected to a Gateway. The echo canceller in the Gateway should compensate for echo however may not operate correctly if mis-configured or if the loss plan is incorrect.
If the problem seems to be associated with a particular telephone handset then it may be due to internal feedback - consider using a different handset.
