HtmlAgilityPack NextSibling.InnerText value is blank

The reason this is happening is that if node is a dt element that is separated from its corresponding dd element by some whitespace, then node.NextSibling is an all-whitespace text node (the space between the </dt> and the <dd>). If you look at it in the debugger, you will see that node.NextSibling's NodeType is HtmlNodeType.Text and not HtmlNodeType.Element.

I suggest creating a convenience method to get the text of a dt node's corresponding dd:

internal static string GetMatchingDdValue(HtmlNode dtNode)
{
    var found = dtNode.SelectSingleNode("following-sibling::*[1][self::dd]");
    return found == null ? "" : found.InnerText;
}

Then you can use it like this:

if (node.InnerText.Contains("Tel:")) { telephone = GetMatchingDdValue(node); }

Here's a breakdown of the somewhat tricky XPath used in my method above:

(a) following-sibling::*

^ Select all elements that share the same parent as the current node and occur after it.

(b) following-sibling::*[1]

^ Select the first node in set (a) (if there are any)

(c) following-sibling::*[1][self::dd] 

^ Select all nodes in set (b) that are elements with the name "dd"

SelectSingleNode() selects the first node in set (c), which should always either be 1 or 0 nodes.

You could most likely get by with just following-sibling::dd or following-sibling::*, but the above path contains safeguards. For example, if for some reason, you had the following XML and your current node was the Tel: element:

<dl>
  <dt>Tel:</dt>
  <dt>Address:</dt>
  <dd>50 Fake St.</dd>
</dl>

following-sibling::dd would give you the result "50 Fake St.", while following-sibling::* would give you the result "Address:". Instead, following-sibling::*[1][self::dd] would select an empty nodeset in this case, so the method would correctly produce an empty string as the result.