When tethering through my (UK) mobile provider, nmap reports closed ports as open

[Jaime T <enopatch () gmail com>]

Hi.

Apologies if this is not the correct list for this, but after an hour
of digging, I couldn't find anywhere more suitable.

As per the title, when I tether through my mobile phone (O2 sim, based
in the UK), nmap tells me that closed ports are open.

Firstly, the control (I'm connected to the internet via my "normal"
fixed-line broadband provider):

$ nmap -p 81 -sS scanme.nmap.org

Starting Nmap 7.40 ( https://nmap.org ) at 2019-02-12 18:00 GMT
Nmap scan report for scanme.nmap.org (45.33.32.156)
Host is up (0.18s latency).
Other addresses for scanme.nmap.org (not scanned):
2600:3c01::f03c:91ff:fe18:bb2f
PORT   STATE  SERVICE
81/tcp closed hosts2-ns

Nmap done: 1 IP address (1 host up) scanned in 0.95 seconds
$ nmap -p 81 -sT scanme.nmap.org

Starting Nmap 7.40 ( https://nmap.org ) at 2019-02-12 18:00 GMT
Nmap scan report for scanme.nmap.org (45.33.32.156)
Host is up (0.20s latency).
Other addresses for scanme.nmap.org (not scanned):
2600:3c01::f03c:91ff:fe18:bb2f
PORT   STATE  SERVICE
81/tcp closed hosts2-ns

Nmap done: 1 IP address (1 host up) scanned in 0.69 seconds

Now, I connect to the internet by tethering through my phone and I run
the same 2 scans:

$ nmap -p 81 -sS scanme.nmap.org

Starting Nmap 7.40 ( https://nmap.org ) at 2019-02-12 18:02 GMT
Nmap scan report for scanme.nmap.org (45.33.32.156)
Host is up (0.17s latency).
Other addresses for scanme.nmap.org (not scanned):
2600:3c01::f03c:91ff:fe18:bb2f
PORT   STATE SERVICE
81/tcp open  hosts2-ns

Nmap done: 1 IP address (1 host up) scanned in 1.05 seconds
$ nmap -p 81 -sT scanme.nmap.org

Starting Nmap 7.40 ( https://nmap.org ) at 2019-02-12 18:03 GMT
Nmap scan report for scanme.nmap.org (45.33.32.156)
Host is up (0.19s latency).
Other addresses for scanme.nmap.org (not scanned):
2600:3c01::f03c:91ff:fe18:bb2f
PORT   STATE SERVICE
81/tcp open  hosts2-ns

Nmap done: 1 IP address (1 host up) scanned in 0.55 seconds

Is this a "known" problem, or is it something more specific to my
mobile phone network provider? In case it makes a difference, I'm
running nmap on debian stretch.

Many thanks, Jaime
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