Error while sending SMS, Phone Number can't be displayed.

• /e/ version:1.6, Android 11
• Device model(s): Samsung Galaxy S8 (dreamlte)
• Device rooted: no

Summary

SMS can't be sent by the default Messages app. Messages coming from other people can still be received. -> This behavior does apply to all contacts, not just ones with a certain device) Also, the field "Status of the SIM-Card" lists the phone number as unknown. (I don't know if these two problems are related in any way)

Possible fixes

ISP (Vodafone) suggests: Alter the SMSC (Short Message Service Center) number...
The only problem is that I couldn't find any option to do this. All documentations I found online suggested altering the number in settings of the Google Messages app in which there is an option to change the SMSC number.