With 2 devices sharing the same line in same partition, you won't be able to call the other line if one is offhook until and unless you assign the CSS correctly to that line.
Dual-line means 2 lines will be available. 2nd line will only be used for Xfer/Fwd/Call Waiting Indications. The dual-line can not be split on 2 phones once one of the lines is in use.
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