The TTL is how they can tell.

If it supports iptables then it you can reset the TTL to 65 at the router so it looks like all traffic is coming from the phone.

It requires root, but this app does the same thing. https://f-droid.org/en/packages/be.mygod.vpnhotspot_foss/

In the Netherlands, regulations forbid the use of cellular phone numbers for non-phone devices, to prevent a future shortage of cellular phone numbers.

Data-only/M2M devices get numbers in a dedicated M2M-only range, and carriers enforce this.

I don't think it's out of the ordinary, sadly. Bell Canada has been known to do just that. They can use the IMEI.