Calls disconnection in between

HI

I am using 2 call servers with IAX . 2nd server calls got disconnected with this message

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[Nov 15 10:03:37] WARNING[7797] chan_iax2.c: Max retries exceeded to host 172.21.0.14 on IAX2/1_1-14973 (type = 6, subclass = 11, ts=60016, seqno=16)
[Nov 15 10:03:39] WARNING[7797] chan_iax2.c: Max retries exceeded to host 172.21.0.14 on IAX2/1_1-7813 (type = 6, subclass = 11, ts=180011, seqno=52)
[Nov 15 10:03:39] WARNING[7799] chan_iax2.c: Max retries exceeded to host 172.21.0.14 on IAX2/1_1-14152 (type = 6, subclass = 11, ts=279979, seqno=83)

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Seems the server didn’t respond to the request but I cannot isolate why it happened.
Any idea possible reason behind it.

Thanks