Hi Luisa,
This can be changed on the Template Level as well, but you will still run into this restriction;
Note: that changes to settings in a template do not affect any of the existing subscriber accounts that were based on that template. This will only take effect for "new" users created after the Template is changed, so you may want to use Bulk Edit.
Subscriber Template Conversation Settings
Identify A Subscriber By Select how subscribers address messages to other subscribers. Subscribers can address messages over the phone by entering subscriber extensions, by spelling their first names, or by spelling their last names. Addressing by name requires lettered keypads on subscriber phones.
In the subscriber conversation, subscribers can switch between addressing by name and addressing by extension by pressing the # key twice. Note that when the Enable Spelled Name Search check box is unchecked on the System > Configuration > Settings page, subscribers can address messages over the phone only by entering subscriber extensions.
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/402/administration/guide/ex/SAG_0140.html#wp1129889
Hope this helps!
Rob