@Jagath Jijo Azure speech service which provides custom text to speech functionality has a free pricing tier with certain limitations on usage. Please check this link for more details. Once the limits are reached you will most probably see your requests fail. You can always upgrade the pricing tier of your speech resource if your volume increases.
The free 12 month Azure trial provides always free services and services free for 12 months. Azure speech service does not fall in either of this category, but you can create an Azure speech resource using the 200$ credit received in your first month or 30days of signing up. This speech resource will be available after 30 days if you upgrade your account from Free to Pay As You Go. If you have chosen free tier plan for your speech resource while creation you can continue to keep the speech resource until you delete the same. Please refer the note at this link for more details.