I have sailed on the Oasis class, and honestly, leaving written messages and leaving phone messages works just fine.
Make a few meet up/check in times each day, and go from there. I personally would leave the small whiteboards inside the cabins, because there have been a few reports of peoples swipping them when they are left on the outside of the door.
We sailed in mid-June and we saw dozens of just handwritten notes on post it notes or similar.
Walkie-Talkies do not work on this size ship, there is too much interference, and even with higher grade ones, it's just too large of an area for coverage.
If you share the Wi-Fi plan, you can send the messages, but until the other person signs on the WiFi they won't get them, so while it is possible, hours could go by until they sign on and see the message, which kinda makes it worthless to do. Even if each person has their own WiFi, the system knocks you off after a certain time of non-use, and you have to sign back on, so.