It is hit and miss. I have had it work, but I've also had it not work. This is the same on other UK networks by my experience too like EE and Three, I'm not sure the exact reasons but I presume it is fussy about network quality, where your IP is etc
Do not rely on it would be what I say. If it works it's a bonus.
If you really need to make lots of calls to/from the UK from the US, get a Three UK PAYG SIM which will definitely work and gives you 3000 mins/texts roaming outgoing and free incoming. Otherwise just keep within the limits of the Lebara bundle.
I would assume on Lebara you will get roaming as per Vodafone UK
US Vodafone:
VoLTE-R and 5G on AT&T
VoLTE-R and 5G on T-Mobile
No connection to Verizon whilst on test, but could be possible
WiFi Call and Backup calling
US Three:
VoLTE-R and 5G on Verizon
VoLTE-R and 4G on AT&T (Steered)
VoLTE-R and 4G on T-Mobile
US O2 Pay Monthly:
5G and VoLTE-R T-Mobile
No connection on AT&T
No connection on Verizon
US O2 Pay & go (Giffgaff and Tesco Pay&Go)
4G on TMobile, 2G calls
No connection AT&T
No connection Verizon
US EE:
5G and VoLTE-R on Verizon
5G and VoLTE-R on AT&T
Unsure of T-Mobile, couldn't test