Cuious, how would phone calls work without a SIM card? Something like WhatsApp I imagine. (Virtual SIM cards anyone?)
Can you explain more what you are asking?
A phone without a SIM can’t get mobile data and therefore can’t use any messaging apps.
As far as I can see, the only way a phone without a SIM can make calls is when it is on WiFi (for those mobile service providers that support that; and also any messaging apps) - and emergency services of course (if it actually works).
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Tracy talks about the eSIM:
As i read this, the Phone have to support this Kind of Chip on its Mainbord. Not sure if someone create a physical Card for eSIM functionality like an upgrade Adapter. I read only about Chips inside of Smartphones like TPM for Example as an Enclave to store that kind of Information. And as i understood the eSIM will be Managed through your Mobile Provider and your Modem-Software so have no free and Open Source feelings here…
Purism however could Build new Hardware and have this Option like the Awesome Simcards too.
However. This IS a virtual SIM Card, so no you have one. And WhatsApp need a Phone Number to Work. However if you own a Contract to a Mobile Provider with some Phone Number you could migrate that old number to another new Phone-Number in EU.
If you like to have some kind of Messaging without Phone Number, try Signal in Future or just XMPP with (E-Mail like) Addresses instead of Phone numbers.
Yes Internet “Phone Calls” based on webrtc are common, and used by many Applications to connect Humans.
Its open for everyone like with Jami. Its just hard to distribute that option because everyone is so focused on closed sources Mainstream options.
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In addition to messaging apps, as @irvinewade mentioned: an option is to add a VOIP account/phone number to the phone and use SIP dialing, either solely over wifi, or in combination with a data-only SIM.
In the U.S., you may get friction from the domestic carriers if you attempt to use a data-only SIM in a telephone, but you can overcome that by using an international data-SIM card designed for travelers (which piggybacks on the local mobile carrier(s)’ network(s)).
Of course, using a data-only SIM is technically not in the scenario you asked about, since it’s in fact a SIM card; it’s just not a normal mobile carrier SIM card that includes a phone number.
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They have. Soprani.ca (who also run JMP.chat XMPP messaging/calling app) have an eSIM adapter capable of loading multiple eSIM profiles from the usual merchant suspects.
I believe there’s another company that offers those, too. (esim.me).
An eSIM can be provided by a mobile service provider, or by a third party, e.g. a company that sells date-limited roaming eSIMs for travel purposes. And yes, you need either a phone that supports eSIMs, or one of the aforementioned adapters.
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Now I’m wondering why my question about a SIM was flagged as hidden as off-topic (albeit temporarily). I didn’t think my post was offensive, unless it was a logic offense.
It was more along the lines of a SIM in a typical linux computer. I think Amarok answered that for me.
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I suspect it wasn’t immediately obvious to the moderator what making phone calls with/without a SIM had to do with flashing a Librem 5 from within a VM. 
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A typical Linux computer would be on the local network (whether via WiFi or via ethernet) and could make phone calls using e.g. VoIP or e.g. any messaging application / web site that makes calls that are adequate for the intended purpose (audio / video). You don’t need a SIM for either of those (and I hope you never do!).
The adapter that @amarok talks about may well be usable in a typical Linux computer but I am not sure that there would be many applications on the computer that could make use of the resulting SIM.
The basic goal and purpose of a SIM card is to … hold an identity that identifies you to a mobile service provider (your Subscriber Identity) and a secret key that authenticates you under that identity to the mobile service provider, and protect the secret key from discovery even by someone in possession of the SIM card, and protect the secret key from theft and/or abuse by using a PIN that allows only limited attempts.
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I agree. Also without the internet, the same linux computer would need to transmit on the phone frequency spectrum to a cell tower or something of that nature.
To call without a SIM you would need to modify the firmware and software the modem runs, so that it provides what ever information you want (1) to the cell tower, not what information the SIM provides? 
(1) - what ever information you want - That’s probably the hardest thing, as you would need to basically hack into phone companies networks.