If your alarm is serviced regularly by an alarm company and you have some type of monitoring on your system, it is unlikely that you will be able to have a different alarm company work on your system.
However, there is nothing to stop you from getting ahead of the game and asking other companies for quotes that include the changes needed to prepare you for Digital Voice The Next Generation of Calling.
Note that companies can require up to 3 months notice to cancel service and monitoring contracts, so make sure to give yourself enough time to get your quotes in, mull them over, and still give notice to your current alarm company if you intend to change. Always choose an NSI approved alarm company to ensure high standards of professional knowledge and ongoing support.