Can T-Mobile Really Distinguish Between SIM Card Usage in a Phone vs. a Router? - eviltoast

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 The only available wired internet is via an outdated telephone line, offering a mere 2mb download speed, which is far from sufficient for my needs.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!

  • RagnarLunchbox@alien.topBEnglish
    2·
    1 year ago

    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!

    • Better_Original_3220@alien.topOPBEnglish
      1·
      1 year ago

      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?

      • af_cheddarhead@alien.topBEnglish
        1·
        1 year ago

        You might want to consider the purchase of a second phone that can be permanently tethered to your router or phone. T-Mobile will definitely be able to detect usage directly in a router SIM slot. A buddy of mine purchase a couple of plans that used a SIM intended for phone plans in the router, these plans were quickly shut down by T-Mobile and/or ATT. He had no recourse to go after the shady dealers as they had disappeared.

      • RagnarLunchbox@alien.topBEnglish
        1·
        1 year ago

        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