Sporadic busy signals when attempting warm transfer of outside calls to internal extensions

Hello, we are using a Poly CCX 600 desk phone with our Zoom Phone service as a receptionist. The rest of the office is using Poly CCX 400's except for 1 other Poly CCX 600 that's in the owner's office. The issue that we're having is that sporadically, when an outside call comes in on the main company number and the receptionist attempts to conduct a "Warm" or "Consultation" transfer to another extension in the office, a fast busy signal is received when the other extension is dialed. However, if you then attempt to do a "Blind" transfer to the same extension, the call goes through. This doesn't happen every time though. It may work 5 times in a row and then may not work a couple of times. Also, when attempting to do the "Warm" or "Consultation" transfers internally from extension to extension there's never a problem. In addition, if an outside call is received on any of the extensions other that the reception desk phone, the "Warm" or "Consultation" transfers work fine. Thank you for any help anyone can give me.