Best practices for transcoding OPUS/G711 using freeswitch

Opensips is used for creating highly scalable SIP signaling routers. So yes, use OpenSIPS with the Carrier Route module to authenticate peers and route calls to the FreeSWITCH boxes.

Handbrake-CLI and inoticoming should be all you need. You'd run inoticoming something like:

inoticoming /my/drop/folder /my/script {} \;

The script would follow the general pattern of:

#!/bin/bash
OUTDIR="/my/output/folder"
INPUT_FILE="${1}"
OUTPUT_FILE="${OUTDIR}/`basename \"${INPUT_FILE}\"`.mkv"
HANDBRAKE_OPIONS="-q 20 --keep-display-aspect"

# Wait for file to be closed
do    
    lsof -n "${INPUT_FILE}" >/dev/null 2>/dev/null
    FILE_STATUS=${?}
    sleep 5
while [ ${FILE_STATUS} -eq 0 ];

# Process the file
HandBrakeCLI ${HANDBRAKE_OPTIONS} -i "${INPUT_FILE}" -o "${OUTPUT_FILE}"

Note that I haven't tested this, so it may require a little bit of tweaking.

If that particular model supports MKV then try that, otherwise I'm afraid I don't know. It would help people answer your question better if you tell us (edit the question) what the devices you're wanting to play the media on, and what other restrictions have have are.