Short answer, at your own risk. The memory card situated in the callers are normally glued in place. Though it is possible to remove them, this normally results in damage to the card when done incorrectly; with a telltale crack running horizontally through the middle of the card.
If a unit is returned because of this, you will be charged for a replacement card and also return postage.
If you are looking to add additional calls to your unit please read our guide for instructions.