Regardless of cabling choice, I tell customers to figure about 98 kbps per Sip call when using G.711.

Double that for remote (internet) calls that need to have remote audio forced through the server for firewall or other issues.

Here are a couple of links to help calculate bandwidth requirements for situations:

https://www.newport-networks.com/cust-docs/52-VoIP-Bandwidth.pdf

https://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.html

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