Dear Stephen,
Well gentleman, before raising a query you could have done at least some amount of Google search or search in this forum to update yourself about the concept of "retrenchment". Learned members of this forum like Mr Umakanthan, Mr B Saikumar, Mr Nathrao etc have given their replies on this subject so many times. There is nothing like "voluntary retrenchment". To improve your knowledge on retrenchment, you may check the following link:
Retrenchment under Industrial Dispute Act, 1947
My reply is based under the assumption that you are from India. If not, then you may have to follow the labour laws of your country.
Thanks,
Dinesh Divekar