how exactly does this setting work?

* if i say keep messages for 30 days, does that mean messages older than 30 days will be removed? or will they only be removed to make room for new messages if i run out of memory space?

* what happens if i say keep messages forever?

* does a memory card come into play here somehow?

thanks

If you set it for 30 days, it will delete messages that are older than 30 days. If you set it for 'forever', it will keep messages until it runs out of memory and then it just deletes stuff at random. I wouldn't recommend setting it to 'forever'. And no, you cannot store the messages to a memory card.

ok but set at 30 days, will it automatically delete anything older than 30 days immediately? or only when it has to make room for new messages?

Yes, it will delete anything over 30 days, no matter how much or how little memory it has.