Welcome, Dear
Thanks for reaching PayTabs out on our Developers Community. My Name is Mohammed ELRayes from the technical support team and I will be looking into this post and following up with you.
Kindly note that, In Paytabs merchant test profiles have SMS credit, This error message is showing up after reaching this limits.
Usually, I would suggest creating a ticket to be handled by the concerned team, I can notice that from your side you already did that, You will have a response ASAP.
Hope that answers your questions
You should also know that we are providing you with a great "Solutions Portal" that includes so many articles that can walk you through any situation such as the "The How to & The What Is (FAQ)" category which contains the technical and non-technical FAQ, and the "How to Integrate with PayTabs?" category which contains all the available methods to integrate with the fully detailed documentation and "Troubleshooting Your Issues/Errors" category which will allow you to trace and solve your issues without seeking for any assistance.
Please don't ever hesitate to comment back if you still have further inquiries. Let me know if there's anything else I can help with.
Glad to be always in help.
Hoping you have a profitable and successful trade.
Lsnav MHN
A response indicating "API: failed to send SMS" when sending POST API request 'Send Invoice SMS'.