Is this happening to all the phones in the site or just 1 or a couple?
If it's the whole site, it may be the actual communication path between the phones and the call manager. I'd check the CCM switch-port config and double-check the VLAN (and Inter-VLAN routing if any) & the QoS.
I've said this because CCM actually interacts with the phone when the call is being setup (off-hook, ringtone, dialling and connecting with the other party), but once the call is connected, it's just an RTP stream between both phones.
If it's just one or two phones, then as dumb as this may sound, I think you should try replacing the phone itself.
I've had that issue in the last couple of weeks at a few sites.
What I found was that the port connecting the handset to the phone itself may be loose, or the port that connects the IP Phone to the switch; so a replacement phone did the job.
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