I did buy a Tablet:
Lenovo Tab M10 FHD Plus (2-nd Gen) ZA5V0183BG

There is:
1 x microSD
1 x Nano-SIM card

My SIM card is inserted into nano-SIM slot ...
Why this tablet does not work as a Smartphone ?

You haven't given us a lot of info and I know next to nothing about tablets. These may be stupid questions but:
1 Does it have a phone app and can you download one if not?
2 Is your sim actually connected to a provider so it can make and receive calls?

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Excuse me, I don't know what to expect about Tablet.
I don't know there exist phone applications that I can download and install.
First the SIM cad was into Smartphone 4" dsplay Lenovo 1000 it worked as a phone from 2016 to 2022 years.
My SIM is actually connected to a provider so it can make and receive phone calls: Some days ago I inserted SIM card into another old Tablet - there it worked as a phone, but did not work with mobile data (internet).
Then I did buy above 10" display Tablet Lenovo Tab M10 FHD Plus (2-nd Gen) ZA5V0183BG and trying to work with phone and with mobile data.

Please suggest me with name of a phone app, because I can download it ( Wi-Fi works with the Tablet ).

"Tablet calling is easy
Once you’ve established an internet connection, you really need only two things to make your tablet function as a smartphone: an app that makes use of VoIP (Voice over Internet Protocol) or VoLTE (Voice over LTE) wireless calling technology, and a pair of headphones.

The former lets you make calls and send text messages over the internet. The latter keeps you from walking down the street with an iPad pressed to your ear, because that would be the opposite of cool.

Here are some popular — and free — apps to consider:

Skype: Download the Skype app to call or message any Skype user for free using your iPad, Kindle Fire HD, Android or Windows tablet. Video calls are also free within the Skype network, but calls to cell phones or landlines are an additional charge.

Facebook Messenger: Facebook is great for keeping in touch with old friends via photos and status updates. But download this app to your tablet and you can text or call anyone who also has Facebook Messenger — you can even place or receive video calls. Messages, voice and video calls are free to anyone, where the service is available. You just need a Wi-Fi or data connection. Facebook Messenger works on iOS and Android tablets, as well as desktop devices.

FaceTime: Apple’s FaceTime app makes it easy to make video calls on an iPhone or iPad over Wi-Fi or cellular data. And the company now lets users make FaceTime calls to those outside the Apple ecosystem simply by sharing a link.

Google Duo: The free app from Google lets you make video and voice calls to other Duo users over Wi-Fi or cellular data. The app works across mobile and desktop operating systems, which makes it a good alternative to Apple’s FaceTime. Note that you’ll need to enter a phone number and have access to a phone that can receive an SMS text message during the initial setup.

This list of apps isn’t extensive. There are other options out there, some free and some that charge a modest fee. Try them out, and pick one that works well for you. Happy (and hopefully cheap) calling!"

https://www.nerdwallet.com/article/f...f%20headphones.

https://www.verizon.com/articles/how...et-as-a-phone/

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Thanks for the advises,
3 hours ago my optical internet was stopped for some unknown reason by my internet provider, so I lost connection to this site from desktop computer I use ....
Then I did try to connect to internet via Tablet - I was surprised - the Tablet had internet access via mobile data from my phone provider.

There is interesting behavior of 2 Tablets:
- old one had phone calls by default 3-4 days ago, but it had not mobile data - internet access (phone provider).
- new one has not phone calls by default, but has internet access by mobile data (phone provider).

Will be easy for me to visit nearest office of my phone provider tomorrow morning and to report them that Tablet can not make phone calls - they will do something to solve my problem.

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If they added in the Google phone then you could make phone calls via your cell plan.
I've never seen a tablet with the phone app installed. I've see phablets but not tablets.

It might be an issue with the the antenna in the tablet not being able to connect to the voice frequency.
An older SIM may use LTE for data, but still use 3G for voice service, and if your tablet does not have the 3G antenna then it can only use data.

If that is the case, then you can use a phone app that will use data for voice calls.

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If it is supposed to have voice then it could require the provider to download carrier services. Just like some phones are able to use wifi calling.

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The advice in post #4 does not utilise the SIM card's built-in phone number.
The answer is simple: If the hardware is there (a SIM card module, a speaker and a mic) all you need is the appropriate software.
Assuming your tablet uses the Android OS, and Android also runs on phones, it should be possible to install something. Minor tweaking might be required - I recommend checking out xda-developers.com and search for "phone calls on tablet" or some such.

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Well it looks your tablet has the protocols for voice. Did you try manually selecting the carrier in the phone settings?

Another thing to check is if the IMEI is corrupted, dial *#06# and make sure that it is not null or corrupted.

This link has steps to test if the cellular radio is turned off and how to turn on, skip down to step 5.
https://www.wirefly.com/guides/andro...ice-how-to-fix

I had a thought about your other tablet that worked with phone, but not data. You may have to manually configure the APN settings to use mobile data. And since you have a tablet that works with data, you should be able to copy its' settings.

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Explanation from the seller:
I returned that tablet back to the seller.
+ € 50 I got a new Tablet which can functioning as "phone" and it works fine using mobile data too.

I have an old Thinkpad with a SIM slot but as I understand it, it needs a WWAN card to connect to cell providers, no?

^ A "WWAN card" is a SIM card. One that provides mobile internet - solely or in addition to telephone - via contract or prepaid model.
Once you got something like that, NetworkManager's MobileManager has a wizard that usually knows how to register it.

Please explain that relationship more fully, if you would, ondoho. The WWAN card is quite large with many connections and an antenna and has no identity that I know of (beyond MAC address) while a SIM card does nothing until activated by buying an account which locks in a specific identity beyond mere MAC address afaik.

FWIW I am a complete N00B at smartphone technology having never owned one before 2 weeks ago. I'm reasonably knowledgeable about typical TCP/IP and UDP networking but Smartphones seem a whole new ballgame. I'm just learning.