As you mentioned phones…you failed to mention which models and firmware.
To be certain of the issue, you should do a network capture at the UCM while making a call that exhibits the issue and post the results. Please mask some of the public IP info, but not all as we need enough of the IP to know if public or private when look at the pcap file.
While I could be wrong on this, CN (comfort noise) is generated when the phone detects silence. In some phones it is tied to the Voice Activity Detection Circuit (VAD). When the VAD does not see activity it then engages the CN. There may be some settings within the phone that you can adjust to see if any change. This function causes RTP packets to be generated so that the other side will know that the connection is alive and won’t end the call prematurely thinking that the loss of packets is a sign of a dropped connection. It is my opinion that you would not hear any noise being generated by the phone as the packets are being sent out. So, there is either CN being generated by the provider, or sidetone or some other issue at hand. It may also be nothing more than the provider’s connection being setup which may involve other carriers beyond them and as they may have early media enabled, you are hearing whatever it is on the other side of your provider and there may not be anything you can do.
However. you mentioned other phones not having the issue,. what make and model are these?