Hello jsenzik123,
the SG300 in regards to port mirroring can only do 1 port to 1 destination, however there is an option to capture a specific VLAN. So you can monitor the Voice VLAN on that particular switch and get all the traffic that is taking place.
For more information here is the datasheet: http://www.cisco.com/c/en/us/products/collateral/switches/small-business-smart-switches/data_sheet_c78-610061.html
SG300 admin guide: http://www.cisco.com/c/dam/en/us/td/docs/switches/lan/csbms/sf30x_sg30x/administration_guide/78-19308-01.pdf
If you have any other questions please let me know.
Hope this helps,
Michael D.
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