Numeric character references

Hi,
I want to understand how the various DOM parsers are expected to behave when
the XML document contains numeric character references. For example, I have
the foll. XML doc:
<?xml version="1.0"?>
<vxml xmlns="http://www.w3.org/2001/vxml" version="2.0">
<form id="phonemedemo">
  <block>
	<prompt>
  		<phoneme alphabet="ipa" ph="t&#x252;m&#x251;t&#x28A;">
tomato </phoneme>
  		<!-- This is an example of IPA using character entities -->
  	</prompt>
  </block>
</form>
</vxml>

Should the DOM parsers convert the numeric character references for "ph" to
something?
If yes, then if I want to conver it back to XML, how should these be
converted back to the numeric values?

Any help or pointers will be very much appreciated.
Thanks,
Roopa