Unfortunately, HughesNet doesn't support WiFi calling, but they do offer their own VOIP service.
With regard to the data, many things can use data without you knowing, and some can use a LOT of it, especially if they aren't set to use as little data as possible. Streaming is usually the biggest culprit, and while someone can stream with HughesNet, it must be done conservatively, and it's best to stream in SD, as streaming in HD can use 3GB or more per hour. And, in reality, you're probably using a LOT more data than you think, due to the fact that, if you are a new customer, you were in a relaxed bandwidth period during your first twenty days. This means that during that time your data was reset on a regular basis, almost like having unlimited data. So, however long it took you to run out of data after that twenty days is about how long it will take you to run out of data during a normal month if things aren't adjusted and/or changed.
What devices do you have connected to the service? Not specifics, but as an example, "a Windows 10 laptop, a Windows 8.1 desktop, two iPhones, and a DirecTV receiver."
With your speed dropping, is it happening primarily in the evening, or is it happening at any time of the day? Do you notice any difference in speed with something directly connected to the modem with a LAN cable vs something connected via WiFi?
Do you have ad blockers installed on your computer browsers? Ads can use a lot of data, and blocking them from running can save that data. One of the most popular ad blockers is Adblock Plus. It's available for Chrome, Firefox, Edge, Safari, IE and even Android. It's free, and if you do a lot of browsing, it can help quite a bit. https://adblockplus.org/
An ad blocker is pretty universal, as most people at least browse, but other things would require knowing what devices you are using (which is one of the reasons I ask what it is you have connected).
With regard to the WiFi calling, if it hasn't been longer than 90 days you can request a review of your sales call, and if it is determined that you were misled you may have recourse. But, with that said, the recourse won't include making WiFi calling work, as it simply doesn't work with HughesNet.
If you're absolutely positive that the only things you have utilizing your HughesNet service is two iPhones and a Windows 10 laptop, then those three devices are where the concentration needs to be regarding your data usage issues, as those are the only three devices that could be using it.
With the iPhones, because I am not very familiar with them, the only thing I think of with certainty is to make sure that you are not connecting to the iCloud, as that can use a lot of data. I'm not sure about updates with iPhones, but you might want to check that, too. Apple products do include an Activity Monitor, and a Network tab within that monitor that can tell how much data each phone is using, but it resets when the phone is turned off, so it can only tell you how much data a phone has used since it was last turned on.
With the Windows 10 Laptop, make sure you aren't using the Cloud (OneDrive). If you haven't set it up, you aren't, but if you have, that can use a lot of data. Also, make sure you aren't sharing your updates with other Windows 10 Computers on the net. For the Fall Creators Update version, go to Settings/Update & Security/Windows Update. Click on Advanced Options (at the bottom of the update options), then scroll down and click on Delivery Optimization. Turn OFF "Allow downloads from other PC's" if it's on. In version 1703, after clicking on Advanced Options, I believe it's under "Choose how updates are delivered" rather than Delivery Optimization.
You've got something chewing through your data, and figuring it out is the tricky part.
As for the speed issues while you still have data available, they reps will need some speed tests to be run to not only verify the speed tests, but also give them information regarding the slow speeds. The protocols for the speed tests are below...
To help the HughesNet Customer Service Reps get a head start on your speed concerns you should create a testmy.net account and, while signed into that account, perform a batch of 3-5 tests during different parts of the day, then share the account results link with us here.
Please keep in mind that HughesNet will only accept testmy.net and the official HughesNet speed test results. Tests from other sites like speedtest.net are not accepted due to the compression technologies and latency that satellite deploys.
Most important points to remember during this test:
-do the tests while directly connected to the HughesNet modem with a LAN cable (NO third party Router or Wireless devices can be used)
-disable the WiFi while the speed tests are being performed
-use the manual 25MB size download test file
-If testing upload instead of download, you must use a 4MB size upload test file
-space each test at least 5 minutes apart
-post your testmy results URL here, it may look something like http://testmy.net/quickstats/yourusername
For a more in depth guide on running the tests, please visit: http://customer.kb.hughesnet.com/Pages/7001.aspx
The Reps are on M-F from approximately 8AM to 5PM Eastern. They will be the ones to address your speed issues, but they will need these tests to do so.
If you are unaware of how to turn off the WiFi in the HT2000W modem, please see 5.c. in the section titled "How do I manage my built-in Wi-Fi modem?" in this PDF. You may have to click Save Settings after unchecking "SSID enable" for each radio, as unchecking all four, then clicking Save Settings, doesn't always save the settings (it doesn't for me). Thus, uncheck "SSID enable" for the 2.4Ghz radio and click Save Settings, then uncheck "SSID enable" for the 2.4Ghz Guest radio and click Save Settings, and so on for the the other two (5Ghz and 5Ghz Guest).