GReminders offers Email, SMS/Text and Phone Reminders as a way to deliver appointment reminders.
We have relationships with Telecom providers across the globe. Each country actually has its own pricing, features and rules for sending and receiving text message, or phone calls.
SMS/Text and Phone are exponentially more expensive to deliver an email. Email is the cheapest form of delivery for us.
So, unfortunately, you cannot substitute your Email Credits for SMS or Phone Credits.
However, you may consider sending Email Reminders a few days prior to your appointment and reserve SMS or text messages for 24 hours or a few hours prior to the appointment. This allows you to progressively adjust your notifications using different channels.