not directly supported by 3CX. you have to manually configure the W56H handset in the T54W web gui. not for novice users.I have that and have tried to use it with a W56H on a T54W phone and it does not work. The cordless handset pairs but it does not get dial tone. 3CX said it's not supported.
Founded in 2005, when VoIP was an emerging technology, 3CX has gone on to establish itself as a global leader in business communications.