Two different products get sold under this heading and they solve different problems. A parallel dialer puts a human rep on more calls. An AI voice agent removes the rep. Buying the wrong one is the most common mistake here.
The number that decides it
The median rep makes about 46 dials a day at a 6.1% connect rate, which is roughly three live conversations. A parallel dialer multiplies dials. It does not touch the connect rate.
So if your list is right, dialers work, and four times the dials really does mean roughly four times the conversations. If your list is wrong, you reach the wrong people four times faster and pay four times the carrier cost to do it.
Aggressive multi-line dialing is also the exact pattern carriers use to flag a number as spam. A flagged number gets labelled on the recipient's handset, which lowers the connect rate you bought the dialer to raise. Rotate numbers and watch reputation, or the gain reverses within a quarter.