Single person version
Salve. Ego, (insert name), absum, quodsi nominem, numerum, atque nuntium tuum dabis, quod primum te vocabo. Gratias tibi ago. Vale.
Multiple roommates version
where n = the number of roommates:
Salve. Nos, (insert name 1)...(insert name n-1) et (insert name n) absumus, quodsi nominem, numerum, atque nuntium tuum dabis, quod primum te vocabimus. Gratias tibi ago. Vale.
Hi. I/we (insert name/s) am/are gone, but if you leave your name, number and a message, I/we will call you right back. Thank you. 'Bye.
If you want to sound like an ancient Roman
If you want to sound like Ecclesiastical, pronounce everything like it's Italian.
I used this all the way through university. Disappointingly, none of my Classics professors left messages in dead languages. The best any of them did was "Erm, hail..."
On the other hand, one of my roommate's teachers rang up, listened to the message and hung up. He then composed a message in Latin, rang back, and read it off. Mercifully for my roommate (a Spanish major), he then left a translation.