If the rate is too low - say less than 3.8% - the investors will cry foul and insist the deal is no longer voluntary.

The sticking point is the interest paid on the new bonds, bearing in mind they will not mature for 30 years. 

If it isn't voluntary then Greece is in default.
 
Gavin Hewitt, BBC Europe editor, 23 January 2012, 10:00 GMT