I tried *135#(simcard number), but it did not work.
OK, so it boils down to how to modem handles the USSD codes.
That isn’t accurate because some codes that are called USSD codes are not USSD codes and because being handled on the handset could mean in the modem but could mean on the host CPU.
Try one of the non-USSD codes listed in the table at: https://en.wikipedia.org/wiki/Unstructured_Supplementary_Service_Data#Code_table (i.e. one of the variations using 31).
I’ve checked the USSD page of my provider (Fonic in Germany). They support the following codes related to switch-off call transfer:
deactivate on no answer: ##61#
deactivate on not reachable: ##62#
deactivate on busy: ##67#
I tried them all with the Call app. After entering the above code and pressing the dial button, a new window appears labeled USSD and a rotating circle, but no other message or visible reaction. But it seems that the switch-off in the network worked.
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I went to the log files and from my attempts this morning it shows:
Jan 5 07:55:57 pureos gnome-calls[1295]: Using origin '4917600000000' for call to '##61#'
Jan 5 07:55:59 pureos gnome-calls[1295]: USSD Error: GDBus.Error:org.freedesktop.libqmi.E
rror.Protocol.SupsFailureCase: Couldn't originate USSD operation: QMI protocol error (92):
'SupsFailureCase'
Jan 5 07:56:48 pureos gnome-calls[1295]: Using origin '4917600000000' for call to '##62#'
Jan 5 07:56:51 pureos gnome-calls[1295]: USSD Error: GDBus.Error:org.freedesktop.libqmi.E
rror.Protocol.SupsFailureCase: Couldn't originate USSD operation: QMI protocol error (92):
'SupsFailureCase'
Jan 5 07:57:27 pureos gnome-calls[1295]: Using origin '4917600000000' for call to '##67#'
Jan 5 07:57:30 pureos gnome-calls[1295]: USSD Error: GDBus.Error:org.freedesktop.libqmi.E
rror.Protocol.SupsFailureCase: Couldn't originate USSD operation: QMI protocol error (92):
'SupsFailureCase'
Time to file a bug issue (will do later).
Thanks Guru.
That is promising in a sense i.e. it is at least attempting to recognise the USSD code and do something with it. 
When I do the same with my beloved Ubuntu BQ E4.5 its dialer app shows:
with this reaction after 12- secs:
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updated
I used a terminal app kermit to talk directly to the modem; dial strings must be terminated by ; to make them work:
purism@pureos:~$ sudo kermit ~purism/.kermrc
C-Kermit 9.0.305 OPEN SOURCE: Alpha.02, 19 Sep 2020, for Linux+SSL+KRB5 (64-bit)
Copyright (C) 1985, 2020,
Trustees of Columbia University in the City of New York.
Type ? or HELP for help.
(/home/purism/) C-Kermit>c
Connecting to /dev/ttyUSB3, speed 9600
Escape character: Ctrl-U (ASCII 21, NAK): enabled
Type the escape character followed by C to get back,
or followed by ? to see other options.
----------------------------------------------------
atd##62#;
OK
^MODE: 3
+DISC: 5,0,0,31,"##62#",129
NO CARRIER
^MODE: 9
...
it also does work to dial a valid number:
atd0170xxxxxxxxx;
NO CARRIER
^MODE: 3
+DISC: 15,0,1,69,"0170xxxxxx",129
^MODE: 9
NO CARRIER
^MODE: 3
+DISC: 15,0,1,69,"0170xxxxxxxx",129
^MODE: 9
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Is Fonic inside of this package: How will APN settings work? Also, thanks for bringing this issue forward!
apt list mobile-broadband-provider-info
apt list prepaid-manager-applet
sudo apt install prepaid-manager-applet
For example, after installation of prepaid-manager-applet might (at least) recognize which provider used there:
As @guido.gunther already wrote to us: “It uses ModemManager (the GSM and 3G helper of NetworkManager) to talk to the modem and mobile-broadband-provider-info to determine the necessary USSD codes for your provider.”
Today I’ve received confirmation that USSD codes aren’t working due to a bug.
Purism support has created a ticket for the issue here
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I stand corrected 
I was a week in Cuba with a cuban SIM card of the provider Cubacel and the USSD code of showing the balance of the account (*222#, see left screen) and buying an additional package from the overall balance (*133#) and the resulting dialog to chose with a number the desired package (see right screen) worked fine. I will try to cut out the syslog of those operations.
Update: There wasn’t anything in syslog about these successful USSD commands.
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Quick questions: What packade is this USSD app and what tweak gets seconds displayed?
I’m not sure if I understand you. One uses the normal gnome-calls app and instead of a normal phone number one uses the dial pad to dial for example
*222#
and presses the the dial button. And the result is what you see in the left picture.
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Sorry, misunderstood the image then - I though that USSD images were some app to use ussd codes. Didn’t realize they are sub-windows to Calls.
The other question, about the seconds, was in reference to the images where the time (top center) has seconds visible. At least from my settings I can’t find that.
The images are just screen shoots of the the sub windows of the gnome-calls app. To get seconds in the top status bar my small booklet says:
33 times displayed and NTP
To show seconds of the time in the lock screen and upper status
bar use:
gsettings set org.gnome.desktop.interface clock-show-seconds true
note: this does not affect the app Clock;
time is synced by NTP, see: /etc/systemd/timesyncd.conf for the config
of the systemd-timesyncd service: (note this shows as comments the
...
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Oh my, that was simple. Thanks!
And also got them on the lock screen too. It’s the small tweaks, that make daily life more convenient 