I have the same problem with a few of my systems connected via ISPs with dynamic IP adresses.
To fix the issue, I was think about using a fake IP address instead of a DDNS name (in SIP and ChanSIP settings). Since there is no external phones, I don't think that it would affect the system.
Would this be a good workaround ?
I know it's dumb, but some customers don't want to pay for a fixed IP.