Hi Rut,
The cause of SMS being stuck in pending could be numerous. One of the first things I would check is the technical workflows to ensure the event status workflow is online and functional. I would then check whether or not you can send a test SMS to yourself. If you are receiving the SMS but the status is still pending in Campaign then reach out to your SMS provider to see if they send a response message back to Campaign.
As mentioned due to the number of moving parts, the reasons can be many, so I would check the documentation below and if you are still stuck, open up a ticket with Campaign Support.
SMS setup and architecture: https://experienceleague.adobe.com/docs/campaign-standard/using/administrating/configuring-sms/sms-p...
SMS troubleshooting: https://experienceleague.adobe.com/docs/campaign-standard/using/administrating/configuring-sms/troub...
Technical workflows: https://experienceleague.adobe.com/docs/campaign-standard/using/administrating/application-settings/...
