Apparently there's not a built-in method to collapse more than one tier of lists using @unionOfArrays.self, but Shane's BridgePlus library has such a method:
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# Requires that Shane Stanley's BridgePlus Library be Installed!
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use AppleScript version "2.4"
use framework "Foundation"
use scripting additions
use script "BridgePlus"
load framework
tell application "Contacts" to tell every group to ¬
set phoneNumList to value of every phone of every person
set phoneNumList to current application's SMSForder's arrayByFullyFlattening:phoneNumList
set phoneNumList to ASify from phoneNumList
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This works quite nicely.