There are many things that could affect the endpoint traffic. You're trying to make a 768kpbs call between the two devices - what bandwidth is available on the connection? What other things are possibly using the available bandwidth? Do you have a QoS policy defined to give priority to Voice and Video traffic? Do you have a firewall that is interfering with the traffic performing and H.323 packet inspection (you should turn the packet inspection off if you do)?
Note also that the C20s are an End of Support device, having passed their end of support date of 31 August 2018. It would be highly recommended that you replace them with newer and supportable devices.
Wayne
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