The manual I downloaded straight from casio is much more thorough than the verizon one, but only talks abut memory getting full or a warning at 80% . by playing a bit im guessing it uses the built in memory of 63mb and after the software and other built in stuff mine has about 27mb left for the ringtones, sounds anything saved to the phone instead of the mem. card. so that being said if you dont have a ton of tones and such 20 some MB should be sufficient. plus I set mine to roll old off as new come in so its not really an issue for me. I do love the message threading according to name. I weekly delete all but one for each person to have a quick reply.