There are couple of possibilities: http://stackoverflow.com/questions/4845660/xsl-how-to-split-strings but the most common is:

    <xsl:stylesheet version="1.0"
     xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:tridion="http://www.xyz.com/tridion/schemas">
    
    
    <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>
    
    <!--Identity transform-->
      <xsl:template match="@* | node()">
        <xsl:copy>
          <xsl:apply-templates select="@* | node()"/>
        </xsl:copy>
      </xsl:template>
    
    </xsl:stylesheet>