If the question is about bandwidth, it is important to know that LiveAgent servers (used for calls) use 64kbps codecs.
So, it's 64 kbps for upload + 64 kbps for download = 128 kbps/sec which is 16 KB/sec.
If we take 20 KB/sec it gives us 1.2 MB/min. With 1GB it gives us 833 min which would be 14 hours.
Based on the question above with 1GB data available, you could be on calls for more than 14 hours.
How it works on the LiveAgent side
VoIP providers that you use in LiveAgent first connect our "call" servers directly, so the agent's internet is not used at this time. Then our "call" servers connect the agent's browser and only this connection consumes the agent's bandwidth.
Is it possible to save traffic?
If you want to save traffic, you can use hardware/software phones and select GSM codec. However, this codec uses compression, so alongside bandwidth consumption, also call quality will be lower!