lua tap and multiple instances of a protocol in one frame

If I understand the problem correctly, you're trying to fetch the last instance of "e164.called_party_number.digits" and "e164.calling_party_number.digits" in an SCCP packet from a tap. The Field extractor fetches all instances of the named field, and they're normally accessed in a for loop. Try this Lua:

local logfile = "CAP_IDP_DATA_"..os.date("%Y%m%d")..".csv"
io.output(logfile)
local called_party_msisdn_extractor = Field.new("e164.called_party_number.digits")
local calling_party_msisdn_extractor = Field.new("e164.calling_party_number.digits")
local sccp_called_gt = Field.new("sccp.called.digits")
local sccp_calling_gt = Field.new("sccp.calling.digits")
– XXX: you don't need these frame fields. You can get them from pinfo.number and pinfo.abs_ts
–local frame_number = Field.new("frame.number")
–local frame_time = Field.new("frame.time_epoch")
local service_key = Field.new("camel.serviceKey")
io.write("FRAME|GT|SKEY|MSISDNs|TIME\n")
local tap_cap = Listener.new(nil,"camel.local == 0")
– Gets the last instance of a Field extractor.
– Returns nil if nothing extracted.
local function last(field)
local last_elem = nil
-- no easy way to get last element of userdata,
-- so iterate the returned values until it reaches
-- the last element
for _,f in ipairs({ field() }) do
    last_elem = f
end

return last_elem

end
function tap_cap.packet(pinfo,tvb)
local calling = last(calling_party_msisdn_extractor)
local called = last(called_party_msisdn_extractor)
local called_gt = sccp_called_gt()
local calling_gt = sccp_calling_gt()

-- XXX: replaced field extractors with equivalent pinfo accessors
--local frame_no = frame_number()
--local frame_t = frame_time()
local frame_no = pinfo.number
local frame_t = string.format("%.9f", pinfo.abs_ts)

local skey = service_key()

io.write(tostring(frame_no) .. "|" .. tostring(calling_gt) .. " -> " .. tostring(called_gt)
    .. "|" .. tostring(skey)
    .. "|" .. tostring(calling) .. " > " .. tostring(called)
    .. "|" ..tostring(frame_t) .. "\n")

end