Modifying the XML element value upon condiion by XSLT - xslt-1.0

Question

I have xml
<PhoneNumberDetails>
<PhoneNumber>
<Number>4162880001</Number>
<TimeStamp>2016-08-16T07:07:44-04:00</TimeStamp>
</PhoneNumber>
<PhoneNumber>
<Number>4162880002</Number>
<TimeStamp>2016-08-16T07:07:44-04:00</TimeStamp>
</PhoneNumber>
</PhoneNumberDetails>
This is input to a XSL. In this XSL i retrieve a phone number from context variable stored in one of the previous XSLT and if it matches to any phone number in above xml, I need to update the TimeStamp to new current Timestamp. I need help in this part.
The XSLT Code:
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:dp="http://www.datapower.com/extensions" xmlns:date="http://exslt.org/dates-and-times" extension-element-prefixes="dp date" exclude-result-prefixes="dp">
<xsl:variable name="currentTimeStamp" select="date:date-time()"/>
<xsl:output method="xml"/>
<xsl:template match="PhoneNumberDetails">
<xsl:variable name="savedPhoneNum" select="dp:variable('var://context/name/phonenum')"/>
<xsl:choose>
<xsl:when test="/*[local-name()='PhoneNumberDetails']/*[local-name()='PhoneNumber']/*[local-name()='Number']=$savedPhoneNum">
<xsl:copy-of select="."/>
</xsl:when>
<xsl:otherwise>
<xsl:call-template name="phonenumber"/>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
<xsl:template name="phonenumber" match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#* | node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="PhoneNumberDetails">
<PhoneNumberDetails>
<xsl:apply-templates select="#* | *"/>
<PhoneNumber>
<Number>
<xsl:value-of select="dp:variable('var://context/name/phonenum')"/>
</Number>
<TimeStamp>
<xsl:value-of select="$currentTimeStamp"/>
</TimeStamp>
</PhoneNumber>
</PhoneNumberDetails>
</xsl:template>
</xsl:stylesheet>
If the phone number does not matches any number in XML i need to add a new element to the above xml and this part works. For e.g.
<PhoneNumber>
<Number>New number</Number>
<TimeStamp>Current TimeStamp</TimeStamp>
</PhoneNumber>

Answer 1

Why can't you do simply:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:dp="http://www.datapower.com/extensions"
xmlns:date="http://exslt.org/dates-and-times"
extension-element-prefixes="dp date">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:param name="savedPhoneNum" select="dp:variable('var://context/name/phonenum')"/>
<!-- identity transform -->
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="TimeStamp[../Number=$savedPhoneNum]">
<xsl:copy>
<xsl:value-of select="date:date-time()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="PhoneNumberDetails[not(PhoneNumber/Number=$savedPhoneNum)]">
<xsl:copy>
<xsl:apply-templates/>
<PhoneNumber>
<Number>New number</Number>
<TimeStamp>Current TimeStamp</TimeStamp>
</PhoneNumber>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>

Answer 2

I suggest you start with something like this. Don't use an absolute path to select the phone number, use it from that context node where the template matches.
<xsl:template match="PhoneNumber">
<xsl:copy>
<xsl:copy-of select="Number"/>
<xsl:variable name="savedPhoneNum" select="dp:variable('var://context/name/phonenum')"/>
<TimeStamp>
<xsl:choose>
<xsl:when test="Number=$savedPhoneNum">
<xsl:value-of select="$currentTimeStamp"/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="TimeStamp"/>
</xsl:otherwise>
</xsl:choose>
</TimeStamp>
</xsl:copy>
</xsl:template>
Note that I assume that you don't have a (non-shown) namespace on the elements of your input xml. If yes, keep your *[local-name()=""] construct, but then the namespace would have to be added to the match="PhoneNumber" and to the TimeStamp output as well.