My apologies for missing out on replying. I checked the Twilio documentation and you are right. Twilio provides you with 15$ credit as a part of the trial account. Once you have used up the credits, you are required to upgrade your account to use their service. Hence in case you want to continue with Twilio, you will have to upgrade to their paid service.
I was also checking for alternatives to Twilio that provide free SMS. However the other servies are similar to Twilio as they provide some credits as a part of the trail account and then you have to upgrade in order to continue using the service. I have listed some of them below for your reference:
1. Plivo https://www.plivo.com/
2. Nexmo https://www.nexmo.com/products/sms
These services provide APIs in various programming languages and you can use them with the Bolt Cloud APIs to complete your project.
If you are looking at sending the alerts to only a known set of devices, e.g. your mobile and your family members mobile, then you could use the Android integration provided by Integromat. Its very easy and quick to set this up. https://www.integromat.com/en/integrations/android/apn. You can use this along with the Bolt integration provided by Integromat https://www.integromat.com/en/integrations/bolt-iot
Integromat provides 1000 operations per month as a part of their free plan. https://www.integromat.com/en/pricing
I hope these suggestions are useful. Let me know if I can help out with anything else.