Yealink's T42/T46 models just only connects up to three participants on a conference. Is there any model or different brand (supported by 3CX) with more participants allowed?
You can try the 3CX client for conference, or you can use Snom phones that make use of the 3CX conference server, passing the load on to the PBX rather than hosting the conference on the device.
Our customer wants to make the conference using directly the phone, others options you said are not valid for them now. I've been seen these options (supported phones):
Yealink (T2X, T4X…) only 3 participants, and CP960, T58A, VP59 up to 5 participants.
Grandstream 16XX up to 3 and GXP17XX y GXP21XX up to 4-5.