@Kiriko0 we had similar situation last week with our guests, they didn't contact us at all for 1 month and then called that they are in front of the building 🙂 We sent them messages on the ABB and they received it as SMS from ABB on their phone. They answered by SMS but of course I didn't get their SMS answer because it was sent to ABB phone number.
Some guests give an email address to ABB but they don't check that email after they book
Some guests can not make international calls with their mobile phones , or just use different phone no while traveling... etc..
But if they don't contact you 1 month before arrival you can contact ABB to cancel their reservation because the worst scenario would be that your guests cancel few days before arrival.