based on: https://stackoverflow.com/questions/4845660/xsl-how-to-split-strings
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:tridion="http://www.xyz.com/tridion/schemas">
<xsl:output method="xml" encoding="utf-8" indent="no"/>
<xsl:template match="tridion:Office/tridion:Telephone/text()" name="split">
<xsl:param name="pText" select="."/>
<xsl:if test="string-length($pText)">
<xsl:if test="not($pText=.)">
<br />
</xsl:if>
<xsl:value-of select="substring-before(concat($pText,','),',')"/>
<xsl:call-template name="split">
<xsl:with-param name="pText" select="substring-after($pText, ',')"/>
</xsl:call-template>
</xsl:if>
</xsl:template>
<xsl:template match="office" >
<h5 id="{@id}">
<a href="#" property="v:name">
<xsl:value-of select="name/text()"/>
</a>
</h5>
<div xmlns:v="http://rdf.data-vocabulary.org/#" typeof="v:Organization">
<div>
<p>
<div class="addressDetails">
<xsl:if test="telephone/text()">
<div class="moreDetails">
<p>
<span>
<xsl:value-of select="utils:Text('Telephone')"/>
</span>
</p>
<p>
<span class="normalfont" property="v:tel">
<xsl:value-of select="telephone/text()"/>
</span>
</p>
<div class="clearfix">
<xsl:comment></xsl:comment>
</div>
</div>
</xsl:if>
</div>
</p>
</div>
</div>
</xsl:template>