Hi Marcello,

Ad Hoc Conferencing

Before Cisco Unified CME 4.1, support for conferencing is limited to three-party ad hoc conference calls using a G.711 codec. To have an ad hoc conference with a party that is not using a G.711 codec, transcoding is necessary.

In Cisco Unified CME 4.1 and later versions, multi-party ad hoc conferences allow more than three parties.

From this excellent CME doc;

Cisco Unified Communications Manager Express System Administrator Guide - Configuring Conferencing

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmeconf.html#wp1010335

Hope this helps!

Rob