The A200 have a trouble with the I.P. changes. When used on “gatekeeper” mode, and the I.P. changes, it tries to re-register without “logging off” first.
That used to be a trouble with older Quintum software.
After version 15 I believe, Quintum changed the gatekeeper software to eliminate the older registration out of the gatekeeper when they saw a newer one from another I.P.
I never tried the DDNS. I could be interesting.
I was reading few days ago on http://www.ddns.org, but I do not see how you will make the A200 register to it.