this post was submitted on 20 Nov 2023
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Can T-Mobile Really Distinguish Between SIM Card Usage in a Phone vs. a Router? (alien.top)
submitted 1 year ago by [email protected] to c/[email protected]
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Hi everyone,

I'm in a bit of a unique and challenging situation regarding internet access and would really appreciate some insights from this community.

I live in an area with limited internet options – no fiber and subpar 5G connectivity. The only available wired internet is via an outdated telephone line, offering a mere 2mb download speed, which is far from sufficient for my needs. My job involves sending and downloading large files regularly, often exceeding 100GB, so a fast and reliable internet connection is crucial.

Currently, I'm using T-Mobile's Mobile Unlimited plan on my phone, which gives me about 50mb download speed. It is called „L Nielimitowana” (I'm from Poland). It's manageable but not ideal, especially since I need to keep my phone constantly connected to my PC. This setup is far from ideal for my work.

I approached T-Mobile about a 5G router plan, but they informed me that they don't offer 5G router plans in areas with suboptimal connections to avoid providing unsatisfactory service. However, they did propose an alternative: getting a second SIM card with unlimited 5G internet at half the price of my current plan. But here's the catch – the representative and the contract clearly state that this SIM card will not work in a mobile router, only in phones.

This brings me to my main questions for you knowledgeable folks:

Is it true that T-Mobile (or other carriers) can distinguish if a SIM card is being used in a router instead of a phone? If so, how do they do it?

Are there any workarounds or specific types of routers that might bypass this limitation?

Having a router with this type of internet would be a game-changer for me, so any advice, insights, or experiences you can share would be immensely helpful.
Thanks in advance!

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[–] [email protected] 0 points 1 year ago (1 children)

Yes, but you can work around any imitations if you are clever and understand their means of detection.

The carrier can tell whether the sim card is in a phone vs tethered to a router and shared. They can track this in two ways.

  1. via the IMEI number of the phone, some carriers will detect if the sim is not in a voice device, so placing the sim in a 4/5G enabled router may be esily tracked & detected.
  2. Tethered to a router via USB can be detetected via the TCP/IP TTL value. With every hop on a netowrk, the TTL of packets is reduced in increments. The carrier can look at the TTL it expects vs the TTL is sees t odetermin that there are more hops downstream of the phone, which will be added by a router.

But this can all be defeated

You simply tether your phone to a router over USB as if it is a WWAN interface. OpenWRT is a great router OS to do this (I have done this with both iphone and adroid, though android is simplest)

AFrom there, with the WWAN setup up, all you need to do is re-increment the TTL by 1 at the router to compensate for the extra hop and your router is invisible to the carrier.

This is done in the OpenWRT firewall custom config: (here is a example I am using in my config, - "wan_iphone" is the usb tethered phone interface name)

iptables -t mangle -I POSTROUTING -o wan_iphone -j TTL --ttl-inc 1

Your carrier wil have no idea!

[–] [email protected] 0 points 1 year ago (1 children)

Hi there,

First off, a big thank you for taking the time to share such a detailed solution! I really appreciate it!

I should clarify a bit more about what I'm looking for, though. My ideal setup is to use a second SIM card directly in a router, without involving my phone at all. This way, I can keep my phone free for regular use and mobility while a separate router manages my home internet needs.

Given your knowledge on the topic, do you think it's possible to set up something like this, especially with the carrier's restrictions? I'm curious if there's a specific type of router that could work around these limitations?

[–] [email protected] 0 points 1 year ago (1 children)

The spare phone as a modem is the only foolproof way unfortunately.

With a router, devices connected behid this creates an an extra hop and this can be detected in the TTL, but you mileage may vary if you put a sim in a WWAN enabled router. It might work or it might not.

I've tried a number of methods with different carriers. Some dont limit this sort of thing via technical surveileance, they just deploy an acceptable use policy. (but these are typically the carriers that give you unlimited data BUT with a maximum speed cap) The faster the link, the more likely you will encounter proactive surviellance.

EDIT: This below usb modem link is also a voice enabled device that you can build a raspi phone out of, and it also can recieive SMS, making it likely it reports as a voice enabled device, which is what carriers usually stipualte. I've had no issues with multiple SIM plans in it.

I use this IOT type usb industrial modem plugged into my OpenWRT router as my failover link which is viraully plug and play in linux, The benefit with this is I can also use this modem on other professional study projects for testing and its much cheaper that a spare phone. Its just another cheaper option to think about.

https://www.waveshare.com/sim7600g-h-4g-dongle.htm

[–] [email protected] 1 points 1 year ago (1 children)

Hi again,

I understand that it might not be feasible to know the specifics of every router model, so I won't list various options. Instead, I'm wondering if it's possible at all for a commercial 5G router, like the Netgear MR5200, to bypass IMEI detection issues. If there's a chance, what specifications or features should I look for in a router to increase the likelihood of it working with the SIM card without detection? If this approach is too uncertain, I'll use a cheap 5g capable phone as a modem instead.

[–] [email protected] 1 points 1 year ago

As I said, the router's WWAN IMEI is a bit like a MAC address that is bound to a hardware brand and type. But unlike simple MAC spoofing, you cant bypass that without some very advanced spy level hack skills and that's can even be a criminal offence in some countries. (IMEI numbers are involved in network and cell tower authentication.) The carrier expects a link with a device that has the IMEI of a voice handset - and the Netgear cant do this.

Your carrier might work with the Netgear just fine, but that will only due to how motivated the carrier is to dectect and police this breach of their terms. You can buy the Netgear, but there is a risk that you'll get cut. The only way to defeat this reliably is via the method I've given you, which is akin to exploiting a technical loophole.

[–] [email protected] 0 points 1 year ago (1 children)

Have you considered spinning up a virtual machine in the cloud and just using your home internet to connect via a remote desktop and do all of your work from there?

[–] [email protected] 1 points 1 year ago

That's a interesting approach. However, I've already invested a lot of money in setting up my home workspace and equipment. I'd prefer to find a solution that allows me to make the most of what I've already established.