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If two phones have the same problem (since they are both IP phones), then you might be having a network throughput problem, in which the bandwidth is intermittently usurped for moments by other network activity, causing the sound to break up.
That's, of course, if the two phones have not been tested upon altogether another network, and if they have both shown the same defect on the same network.
Another test would be, to completely shut down all other possible demands upon the network bandwidth the phones are using... and see if that cures the problem (if it does cure it, then the problem is not in the phones, but in the network.)
Let me know what you find out, and how it goes, please.