How can I increase the SMS memory on the Blackberry 8100?

The micro SD memory card doesn't have any relationship to your SMS messages. They go on the on-board memory of the BB device. If you have a lot of applications running on your phone they will eat up your available memory and might give you a problem with your text messages...but it isn't likely that you have that many applications.

How much memory do you have active on your phone? If it is less than 9MB then you should reboot your phone by pulling out the battery for 30 seconds and allowing it to reload. Then check your memory. Hopefully you will have over 15MB.

To check memory press the BlackBerry key (the one with the 7 dots)from the Home screen, then the "Settings" key then the "Options" key then press "Memory". Good luck!

THANK YOU