Greetings.
In the VoIP (Voice Over Internet Protocol) world, the setting you are talking about is sometimes called "comfort noise", sometimes called "silence suppression".
Most VoIP clients (Skype is a VoIP client) have some settings that allow the substitution of silence (or near-silence) to save bandwidth. Look for the keywords mentioned above, and you may find the settings you are looking for.
'73
- Mike S.
KA9CQL
Victorville, CA, USA