I purchased this in Feb and it worked great. Purchased again Oct 1st and having phone call issue. Have not and don't know about volte process
@Clara5K , it is extremely probably that your phone has ended up falling off of the VoLTE whitelist. The fact that you are connecting to AT&T and having zero luck with calling means you are not able to establish a VoLTE call. Only T-Mobile has 2G service which can also be used for calling. No US carrier has a functional 3G network anymore. If you have purchased roaming data, try using a VOIP app to make calls.
What is the make and model of the phone? Talk is the problem part. Either this place has enabled your phone for volte or you need to be able to switch down to 2G GSM. The latter only works on T-Mobile.
Critical part to US roaming depends on T-Mobile/AT&T coverage and whether you can connect successfully onto either network.
If the phone is not connecting to T-Mobile/AT&T, change the Network Connection from Automatic to Manual and select and test both T-Mobile and AT&T and see which one you can connect.
With your voice issue, make sure VoLTE is enabled on the device setting. And if it does not work, try to change the Network to T-Mobile ONLY, and change Preferred Network Type or Network Mode to 2G and see if the calls go through.
Also, you can download VoIP app TextNow as a workaround to make calls and text if you have Wifi or if your roaming has data component and data is working