Since all U.S. carriers shut down their 3G networks a couple of years ago, and 2G before that (except for T-mobile, which still retains limited 2G coverage), they require Voice Over LTE (VoLTE) to be operational on any device they activate. With no 3G networks remaining, devices that are unable to make Voice-Over-LTE/4G calls have no network to fall back to for calling (with the exception of 2G on T-mobile).
AT&T calls its particular VoLTE implementation “HD Voice” and takes it a step further by actually publishing a limited list of devices (PDF) that they will allow to operate on their network. Anecdotally (from comments in their support forums) they are known to disable call functionality for unapproved devices they detect on their network. They are able to spot an unapproved phone via the device’s IMEI, which is always observable to any network.
Verizon is similarly restrictive, even if they don’t publish an actual list.
T-mobile is more accepting of VoLTE-capable devices that were not originally branded (and/or locked) to their network. T-mobile is not known to disable calling when they detect an already-activated SIM in a non-VoLTE device on their network, but they (likely) won’t activate it for new service. As they still have limited 2G/GSM as a fallback, a customer with such a device may still be able to make calls, but may have frequent connection issues.
Although Purism’s AweSIM/SIMple plans, through an intermediary provider, now use AT&T’s network for service for the Librem 5 (and/or other devices if desired), they are able to bypass AT&T’s usual restriction.
All of the Librem 5’s BM818 modem variants do have VoLTE functionality, and in the U.S. the BM818-A1 is easily activated on T-mobile’s network, or with MVNOs that use T-mobile’s network for service. Additionally, at least one forum member here reported success with using an already-activated SIM card from Pure Talk, an MVNO on AT&T’s network, which is an interesting exception.
It’s unlikely that you will be able to use AT&T itself for the foreseeable future.
(Note that having 4G data is not the same as having Voice over LTE/4G.)