The phones periodically send a TCP ACK ping to
Freeswitch which goes unanswered and then phone then tears down the TCP
connection. I'm not at all a TCP expert but should Freeswitch be
responding to this unsolicited ACK?
I've been working with TCP for 25 years, and this is the first I have heard of ACK ping. A quick Google search told me that this technique involves sending a normal TCP ACK packet (which all TCP packets except the initial one have, by the way) to a random TCP port. The host will then respond with a RST packet. (Which means reset the connection because the ACK was not sent to an established connection.)
However, a firewall may filter and discard these random ACK packets. Since they're not part of "normal" TCP, no one cares.
In answer to your question, this is in the TCP layer much deeper than Freeswitch. There is nothing Freeswitch can do at this level.