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!

Oh yeah I forgot to ask, do you use iphone or android?

If you use iphone you need to run a little script in the background to stop the phone asking for you to trust the connection and stopping the link every so often. I’ve added it for others that might search this solution later…

#!/bin/sh
# Make iPhone tethering stay alive on OpenWrt

# After you successfully trusting the iPhone for tethering, copy files with name like
# /var/lib/lockdown/12345678-9ABCDEF012345678.plist to /etc/lockdown/locks.
# That way, you won't have to set up trust again after router reboots.
if [ -e /etc/lockdown ]
then
mkdir -p /var/lib/lockdown
cp -f /etc/lockdown/* /var/lib/lockdown/
fi

# lockdown records restored, now we can launch usbmuxd. Don't launch it sooner! (this is run from inet.d)
usbmuxd

# We are up and running now. But unfortunately if your carrier signal is weak, iPhone will
# drop connection from time to time and you'd have to unplug and replug USB cable to start tethering
# again. Script below automates that activity.

# First wait a bit - we just brought the interface up by usbmuxd
sleep 60

# If we see iPhone ethernet interface, try to ping iPhone router's address (172.20.10.1).
# When the ping is unsuccessful, rebind iPhone ethernet USB driver and wait for things to settle down
while :
do
for i in /sys/bus/usb/drivers/ipheth/*:*
do
test -e "${i}" || continue
ping -w 3 172.20.10.1 &> /dev/null
if [ "${?}" -ne 0 ]; then
echo "${i##*/}" > "${i%/*}"/unbind
echo "${i##*/}" > "${i%/*}"/bind
sleep 15
fi
done
sleep 1
done
EOF

Yes they can. Based on the imei

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.

Try it, I bet it will work anyway (it does in the US)