Message credits in monthly plans do not rollover at the end of your monthly billing cycle. Monthly credits will replenish back according to your plan at each billing cycle.
Pay As You Go message credits (one-time message credit purchases) do rollover each month, and never expire. Pay As You Go message credits are also the last credits to be used each month, meaning that your rollover messages last longer. You can purchase Pay As You Go credits from your Settings > Plan.