The TA600 Series does not support VoIP SIP. It does support VoIP MGCP. The VoIP MGCP does support G.711. I have not seen any reference to T.38 and MGCP, only T.38 and SIP, so I would say the TA600 does not support T.38. Also, it does not show up in a search of the TA600 manual.
You would need the TA924 to support VoIP SIP and T.38.