The free version of Talkatone is good for this, I’ve been using it for a few years. The interface on the free tier is a bit cluttered but for my very occasional use it is more than sufficient.
I also used it a few years ago when visiting the US to make some calls to local numbers (eg. phoning a shop to check if they had something in stock). That was also free and worked over mobile data which I had from a Three ‘roam like home’ PAYG plan, so allowed me to avoid expensive roaming charges.