It should work for soft phones the same as for desk phones. As per the docs:
// For the sake of this example we'll assume that we have 3 available lines [1111, 1112, 1113]
var telephonyLines = deskphone.lineDirectoryNumbers;
// We'll select line 1112.
var telephonyLineToSelect = telephonyLines[1]
deskphone.selectLine(telephonyLineToSelect);
You should be able to use softphone.selectLine instead, since both softphone and deskphone extend TelephonyDevice but since only deskphone is documented, it may not be available for softphone. I'll check with the Jabber developers.
Based on the sample app, it looks like it does NOT work for softphones. There's no option to select a line when you select the softphone, but there is an option if you select a deskphone.
