I'd probably text if you're not too close lol. Something weird and unusual to generate a reply, something like...
"I hope your Birthday... well, I hope it's not awful. Getting old now, we'll have to go play bingo some time
" (or whatever old people play where you're from) ....I suppose you could use the line about going for a bowl of cereal, if shes a lot younger than you. I usually say something completely non-Birthday related, on purpose. It's a risky move (they're busier than usual and might not reply if they're not too interested) but if they reply, they're usually chasing after a "Happy Birthday" right from the start lol. I literally act like I don't even know it's their Birthday.