This is the config I use for the Grandstream HT802 for faxing which is basically functionally the same as the Ademco communication protocol.
The two most important settings are likely the T.38 passthrough and disabling the echo cancellation.
Create filename cfgabcdefabcdef.xml in the /tftpboot folder of your FreePBX instance. Where abcdefabcdef is the MAC using lower case letters in the filename and uppercase letters in the config file below.
<?xml version="1.0" encoding="UTF-8"?>
<!-- HT8XX XML Provisioning Configuration -->
<gs_provision version="1">
<mac>ABCDEFABCDEF</mac>
<config version="2">
<!-- # Admin password for web interface -->
<P2>domain$upport123</P2>
<!-- # Firmware Upgrade and Privisioning. 0 - TFTP Upgrade, 1 - HTTP Upgrade, 2 - HTTPS Upgrade. -->
<P212>2</P212>
<!-- # HTTP/HTTPS User Name -->
<P1360>123456</P1360>
<!-- # HTTP/HTTPS Password -->
<P1361>0987654321cba</P1361>
<!-- # Firmware Server Path -->
<P192>pbx.domain.com:1443</P192>
<!-- # Firmware File Postfix -->
<P233>_1.0.17.5</P233>
<!-- Config Server Path-->
<P237>pbx.domain.com:1443</P237>
<!-- # Config File Postfix -->
<P235></P235>
<!-- # Automatic Upgrade. 0 - No, 1 - Check daily, 2 - Check weekly, 3 - Check every () minutes. Default is No. -->
<P194>1</P194>
<!-- # Automatic Upgrade. Daily at hour(0-23) -->
<P285>18</P285>
<!-- # NTP Server -->
<P30>us.pool.ntp.org</P30>
<P64>CST6CDT</P64>
<!-- DHCP Hostname -->
<P146>ext123</P146>
<!-- FXS PORT 1 Settings -->
<!-- # Account Active. 0 - No, 1 - Yes. -->
<P271>1</P271>
<!-- # SIP Registration. 0 - No, 1 - Yes -->
<P31>1</P31>
<!-- # Primary SIP Server -->
<P47>pbx.domain.com</P47>
<!-- # SIP Transport 0 - UDP, 1 - TCP, 2 - TLS -->
<P130>2</P130>
<!-- # SRTP Mode 0=Disabled 1=Enabled but not forced 2=Enabled and forced -->
<P183>2</P183>
<!-- SIP User ID -->
<P35>123</P35>
<!-- Authenticatoin ID -->
<P36>123</P36>
<!-- Authentication Password -->
<P34>hexpasswordfromextensionpage</P34>
<!-- Name -->
<P3>Fax 1</P3>
<!-- # SIP Registration Failure Retry Wait Time upon 403 Forbidden default 1200-->
<P26002>3600</P26002>
<!-- # 0 - PCMU, 8 - PCMA, 4 - G.723, 18 - G.729, 2 - G.726-32, 97 - iLBC, 123 - OPUS， 9 - G722. -->
<!-- Codec Choice 1 -->
<P57>9</P57>
<!-- Codec Choice 2 -->
<P58>123</P58>
<!-- Codec Choice 3 -->
<P59>0</P59>
<!-- Codec Choice 4 through 8 -->
<P60>0</P60>
<P61>0</P61>
<P62>0</P62>
<P46>0</P46>
<P98>0</P98>
<!-- Fax Mode 0=T.38 1=Passthrough -->
<P228>1</P228>
<!-- Disable Line Echo Canceller (LEC). 0 - No, 1 - Yes -->
<P824>1</P824>
<!-- FXS PORT 2 Settings -->
<!-- # Account Active. 0 - No, 1 - Yes. -->
<P401>1</P401>
<!-- # SIP Registration. 0 - No, 1 - Yes -->
<P731>1</P731>
<!-- # Primary SIP Server -->
<P747>pbx.domain.com</P747>
<!-- # SIP Transport 0 - UDP, 1 - TCP, 2 - TLS -->
<P830>2</P830>
<!-- # SRTP Mode 0=Disabled 1=Enabled but not forced 2=Enabled and forced -->
<P443>2</P443>
<!-- SIP User ID -->
<P735>321</P735>
<!-- Authenticatoin ID -->
<P736>321</P736>
<!-- Authentication Password -->
<P734>hexpasswordfromextensionpage</P734>
<!-- Name -->
<P703>Fax 2</P703>
<!-- # SIP Registration Failure Retry Wait Time upon 403 Forbidden default 1200-->
<P26102>3600</P26102>
<!-- # 0 - PCMU, 8 - PCMA, 4 - G.723, 18 - G.729, 2 - G.726-32, 97 - iLBC, 123 - OPUS， 9 - G722. -->
<!-- Codec Choice 1 -->
<P757>9</P757>
<!-- Codec Choice 2 -->
<P758>123</P758>
<!-- Codec Choice 3 -->
<P759>0</P759>
<!-- Codec Choice 4 through 8 -->
<P760>0</P760>
<P761>0</P761>
<P762>0</P762>
<P814>0</P814>
<P815>0</P815>
<!-- Fax Mode 0=T.38 1=Passthrough -->
<P710>1</P710>
<!-- Disable Line Echo Canceller (LEC). 0 - No, 1 - Yes -->
<P825>1</P825>
</config>
</gs_provision>