said by achuchma 
:Do keep in mind when you use the POTS network in the US, voice data is usually transmitted on private, dedicated fiber trunks with no other traffic other than voice traffic. If mixed with data, voice will get the highest QoS.
VoIP runs on the "best effort" Internet, which is with a pot luck of other data...
Actually that was accurate back in 1996 or so. Today most carriers are using VoIP on their own network. It allows for more statistical multiplexing (ie: oversubscription) than traditional TDM voice carriage does.
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