We have a customer who has a NEC DSX-40, they have an SLT extension (350, durafon) that rings when calls come in on the line, but when someone tries to intercom call, or transfer a call, it goes straight to voicemail.

I can make intercom calls via the handset as well (though I have to wait a long time for the tone to change before it will go through)

I'm not an expert on the NEC or durafon system by any means, so I'm hoping someone here can help me find what setting on either the durafon or the phone system is not correct. The biggest thing that we really need to figure out is the transferring/intercom.