have you read some of the US/CAN VoLTE threads for Samsungs? if call setup takes long, I guess it’s dropping to 2g. If you don’t do workarounds (your own VoIP over LTE) then you’d need to have a phone with working VoLTE within customroms… see those reports: [LIST] Features list for devices
You can check the 4g vs 2g assumption at cellmapper.net - if 4g coverage is ~okay, no need to wait so long.