Freeswitch responds after the reinvite to T38 with:
SIP/2.0 488 Not Acceptable Here.
Via: SIP/2.0/UDP 10.255.3.100:5060;branch=z9hG4bK-00104-Elh75avx0E4AG5U-0.
From: "Anonymous" <sip:[email protected]:5060>;tag=Elh75avx0E4AG5U-IPTrunk-623-19-17at10.255.3.100.
To: <sip:[email protected]:5060>;tag=cg7Dg06ZXSZ6H.
Call-ID: [email protected].
CSeq: 104 INVITE.
User-Agent: FreeSWITCH-mod_sofia/1.6.14+git~20170209T234209Z~de2936af46~64bit.
Accept: application/sdp.
Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, REFER, NOTIFY.
Supported: timer, path, replaces.
Content-Length: 0.


dialplan:
                <action application="answer" />
                <action application="playback" data="silence_stream://2000"/>
                <action application="set" data="fax_enable_t38=true"/>
                <action application="rxfax" data="/tmp/FAX-${uuid}.tif"/>
                <action application="hangup"/>


spandsp.conf
    <fax-settings>
        <param name="use-ecm"           value="true"/>
        <param name="verbose"           value="true"/>
        <param name="disable-v17"       value="false"/>
        <param name="ident"             value="SpanDSP Fax Ident"/>
        <param name="header"            value="SpanDSP Fax Header"/>
        <param name="spool-dir"         value="/tmp"/>
        <param name="file-prefix"       value="faxrx"/>
    </fax-settings>

freeswitch.log shows:
4b17c550-ef4b-11e6-8931-95b3e9bcbe5b 2017-02-09 22:42:15.398075 [DEBUG] switch_core_media.c:3985 sofia/external/[email protected]:5060 T38 ACCEPT on request
4b17c550-ef4b-11e6-8931-95b3e9bcbe5b 2017-02-09 22:42:15.398075 [DEBUG] switch_core_media.c:4090 sofia/external/[email protected]:5060 T38 IS NOT POSSIBLE on request
4b17c550-ef4b-11e6-8931-95b3e9bcbe5b 2017-02-09 22:42:15.398075 [DEBUG] sofia.c:7950 Reinvite resulted in codec negotiation failure.
4b17c550-ef4b-11e6-8931-95b3e9bcbe5b 2017-02-09 22:42:15.398075 [DEBUG] sofia.c:7035 Channel sofia/external/[email protected]:5060 skipping state [ready][488]
4b17c550-ef4b-11e6-8931-95b3e9bcbe5b 2017-02-09 22:42:15.418078 [DEBUG] sofia.c:7042 Channel sofia/external/[email protected]:5060 entering state [received][100]