Somehow I blocked one number on outgoing calls and I cant seem to unblock it.

that number can phone me, we can text, and we can make Whatsapp calls both ways. I had my service provider have a look on the line, but no issue there.

  When I placed my sim card in different phone I was able to phone the number. The 'block' is therefore not on the sim or at the service provider, but somewhere on the phone.

I am out of ideas where to look!!!

Somehow I blocked one number on outgoing calls on Samsung Galaxy Note 9 and I can't seem to unblock it. That number can phone me, we can text, and we can make WhatsApp calls both ways. 

I had my service provider have a look on the line, but no issue there. When I placed my SIM card on a different phone and I was able to phone the number. The 'block' is therefore not on the sim or at the service provider, but somewhere on the phone.

How to unblock them?