The capture groups in sub will target your desired output:
sub('.*\\((.*)\\).*', '\\1', str1)
[1] "123-456-789"
Extra check to make sure I pass @akrun's extended example:
sub('.*\\((.*)\\).*', '\\1', str2)
[1] "123-425-478" "123-423-428" "123-423-498" "123-432-423" "124-233-848"
