Called back, got recorded message telling me "this mailbox is full. If you are a subscriber, press star to log in. If you wish to leave a message, please try again later"

Yes, completely logical. My automated phone detector thing assumed "Suspected spam - reported as debt collector". I do not have have any outstanding debts that I am aware of.

nuisance number

I had a missed call from this number. I didn't call back. My phone could not identify where the call originated from, just said "Unknown". It's a mobile so if it is important, can contact me via SMS. No SMS left

Call twice in 2 days. Did not leave a message.