I found:

"WILL MY EXISTING SIM CARD WORK ON THE LIBREM 5?

We aim to support 3G and 4G for the most common international frequency bands and carriers. Exact specifications will follow."

Unfortunately, all my contacts communicate via regular phone numbers, so this is a feature that I need for a phone. "We aim" is not too reassuring.

I think the "we aim" here refers to 3G/4G. The phone is going to have a baseband module, so yes, it will be able to do GSM phone calls & SMS like any old phone. I imagine that adding 3G/4G packet data in a FOSS manner may be harder however.

I think the "we aim" bit is more directed towards certain frequencies rather than "we're not sure if we can get 4G working". Take a look at any group of phones on https://willmyphonework.net and look at the differences between countries. Getting 3G/4G to work globally is a lot harder than just settling for US/Canada or US/Europe.