No one else seems eager to reply so here's my take on this question.

It seems that the esim provider negotiates what services they will offer from each carrier. Flexiroam now offers esims for data as well as their original sim stickers and also nano sims I believe. The original Sim4Crew had voice service available, but I don't know if that is still true. Their website has changed, and is a little confusing to me. I'm hoping the 3 UK network goes to the esim option soon, as they will probably have the voice, text and data package available, just like their triple cut sim cards. Basically, you'll need to decide what you need for travel and then maybe just ask on this or other websites where to find it at a good price & service.