Unless all your devices are doing heavy downloading all at the same time, your test speed shouldn't change much. And that would include the phones operating on Wifi and not 3G or 4G. Seeing a momentary drop in speeds every once in a while is normal, but not from 40 to 6. Dropping down by 10-20% would be more normal every once in while. Keep calling them if the problem doesn't go away. Be polite but be persistent. I assume that your speeds become low on all devices.
You could try plugging directly into your modem with an Ethernet cable and see if the erratic behavior is still there. If it's not, then you may have an issue with your your wireless router (if you have one).
