Since version 4.5x you may use the IF ELSE Statement
In the WYSIWYG editor:
{if({Mobile Number})} Mobile: {MobileNumber}{else}Tel: {Telephone}{endif}
This statement will use the telephone text heading and field if the Mobile Number field is blank.
See also the Exclaimer Top Tip document: Designing an email signature (PDF) 104KBNEW
publsihed at http://ww.exclaimer.com documentation area
Hans Willi
Hans Willi Kremer