Contact names not shown in iPhone in SMS?

Let’s say I have a person saved in the Contacts with the national code (+386 for Slovenia), ie +386 31 100 000. When that person calls me, it displays the number 031 100 000, no sender, but displays the name in the SMS i receive from this person. If I save the second number (w/o the national code) in the contacts, there are no problems seeing who’s calling, but the SMS sender is just a number (written w/ the national code). Is there a fix or am I supposed to save two numbers for each contact, one with the national code and one without?