E' più "telefonosa"... 
...On a Windows Mobile Standard (Smartphone) device you will be able to click on the telephone number on the device and dial the number (see below)
On a Windows Mobile Professional (Pocket PC) device you can't click on the telephone number... There is a workaround to fix this and that's to put the Telephone Number in the body of the meeting request and prefix it with Tel: For Example - Tel:+441234567890
See the example below on a Professional device!
Unfortunately this isn't fixed in Windows Mobile 6.1 but you can bet I'm pushing hard on our Product teams to fix this inconsistency.