Having Issues with the telephone function on Samsung S9+

Dear Communitiy,

first of all a happy new year to all of you =)

My “telephone function” on my Samsung Galaxy S9+ is not working properly. Most of the time, I cannot set up calls or don´t receive any calls. I already changed my telephone provider, but stayed in the same telephone net (Telekom). This did not change the issue.
I also checked if other apps would solve this issue, but they didn´t either.
I do not remember very well, but I think the telephone functionality was “buggy” from the beginning. I bought the phone a few months ago as refurbished.
Today I put my Sim card in another phone (Fair Phone 4) and I could set up calls normally. This leads me to the assumption that there is an issue with my phone.

Do you have any suggestions on how to repair/fix the “telephon function” on my phone?
Is there any possibility to check/identify if this is a software or hardware issue?

Very much appreciate your answers in advance =)

Hard facts:
Samstung Galaxy S9+ (bought as refurbished a few months ago)
/e/OS Version 1.6
Provider: Telekom (Germany)

Dear all,

I solved the issue with setting the phone back to factory settings.
So, most likely this was a software issue. I think it is also possible, that old hardware caused this issue.