I got it working! The goal was to run a script to inform users via a text message when they get a voicemail.

This script looks for a "vm-notify-sms" param in the user's directory information. It sends the text from user's outbound caller-ID number, which is set in the standard FS configuration. I'm using the SMS API from Flowroute.com.

--[[ This script is called by configuration in autoload_configs/lua.conf.xml.

     It is called on a "vm:maintenance" event.

]]

-- The following is specifc to the Flowroute SMS API,

-- documented at https://developer.flowroute.com/docs/messaging

url = "https://api.flowroute.com/v2/messages"

authentication = "user:password"

action = event:getHeader("VM-Action")

if action ~= "leave-message" then return end

user = event:getHeader("VM-User")

domain = event:getHeader("VM-Domain")

caller_id_name = event:getHeader("VM-Caller-ID-Name")

caller_id_number = event:getHeader("VM-Caller-ID-Number")

--[[ We don't care about these:

file_path = event:getHeader("VM-File-Path")

flags = event:getHeader("VM-Flags")

folder = event:getHeader("VM-Folder")

uuid = event:getHeader("VM-UUID")

--]]

message_len = event:getHeader("VM-Message-Len")

timestamp = event:getHeader("VM-Timestamp")

api = freeswitch.API();

from = api:execute("user_data", user .. "@" .. domain ..  " var outbound_caller_id_number")

to = api:execute("user_data", user .. "@" .. domain .. " param vm-notify-sms")

if to == "" then return end

if caller_id_name == ''

        or caller_id_name == 'UNKNOWN'

        or caller_id_name == 'UNASSIGNED'

        or caller_id_name == 'WIRELESS CALLER'

        or caller_id_name == 'TOLL FREE CALL'

        or caller_id_name == 'Anonymous'

        or caller_id_name == 'Unavailable'

        then caller_id_name = nil end

if caller_id_number == ''

        then caller_id_number = nil end

message = "Voicemail "

if caller_id_name

        then message = message .. "from " .. caller_id_name .. " (" ..  caller_id_number .. ")"

        elseif caller_id_number

        then message = message .. "from " .. caller_id_number end

message = message .. " at " .. os.date("%I:%M %p", timestamp)

message = message .. " length " .. message_len .. " seconds"

message = message .. " to box " .. user

message = message .. "."

-- Blindly send the text

data = '{ "to": "' .. to .. '", "from": "' .. from .. '", "body": "' .. message .. '"}'

--[[

     Until either Freeswitch has the ability to pass authentication data

     to the mod_curl API (FS-9223), or the Flowroute API has the ability

     to take authentication data in the POST body (they are working on it),

     we are forced to spawn a shell to use the curl from our OS.

]]

-- api.execute("curl", url .. " auth " .. authentication .. " post " .. data)

os.execute("curl -u " .. authentication .. " -H 'Content-Type: application/json' -X POST -d '" .. data .. "' "  .. url)