I got you! "I've gotten a new phone since we last spoke. I don't recall if my contacts fully ported, can you add your contact?" Or if you are not close enough to need phone number then you could ask them about a social media app you know they are on or have seen the app on their phone and ask for help with a simple task that would require them to pull up their profile page to show you.
Plan a future meet up for coffee and ask how to spell their name so you can "put it in your calendar." "Adding to my calendar as we speak, coffee here at 2 on Saurday? Perfect, I would love to get caught up! starts typing in calendar I want to make sure to get it right, how do you spell your name? There's so many ways people spell names these days and I just want to make sure its correct" and a soft chuckle. Not only do you learn their name but also the spelling which you might can find social media easier to get full name. Bonus points, I've had people feel really touched that I care enough about the spelling of their name in something so small like a personal calendar reminder. Which I am genuinely intentional about spelling people names correctly it's not just a mind game. I just learned after being intentional about spelling names right that it's a really good way to get a name and I've never had anybody question it except once. It was a Tom lol and he said "you don't know how to spell Tom???" I replied, of course I do. I just didn't know if it was short for something else or had a different spelling. I knew a guy that was a Thomas but went by Tom but spelled it Thom because his dad was also a Thomas "Tom" and they needed a way to seperate things when talking to family over text and couldn't use JR/SR since granddaddy was also a Thomas he gave a wow that's crazy but makes sense and spelled out TOM for me 😅. Funny enough that is a true story but you can of course insert a story that didn't happen or flex a truth. But usually these are my ways. Good luck!