Link to the page about the phone system rather than guru3.

Fixes #1616

templates/cfp/finalise.html

@@ -20,9 +20,9 @@
           once we're on site. It will be treated with strictest confidence
           and only accessible to those that absolutely need it.</p>
         {{ render_field(form.telephone_number, 9) }}
-        <p>If you have a <a href="https://guru3.eventphone.de/">custom extension</a>
-          (e.g. DECT) issued for our on-site phone network, we'll use that as an
-          alternative way to contact you.</p>
+        <p>If you have a custom extension (e.g. DECT) issued for
+          <a href="/about/phones">our on-site phone network</a>, we'll use
+          that as an alternative way to contact you.</p>
         {{ render_field(form.eventphone_number, 9) }}
     </fieldset>