Xseries :: Can't Store A Number On Phone Memory X3-02

I have a Nokia x3-02 and I detected a strange behaviour when I try to store a number in the phone memory.I try to explain with a example:If you have stored the number 1234567 in the phone memory with the name USER1.Then I want to store another number 661234567 ( the two first are different, but the last seven numbers are equal ) with the contact name USER2, the phone give a message that the number is in use for USER1.It seems that only the last 7 digits are evaluated to determinate is the number is the same in this phone. I been tried this on a 2730 and the result is the same too.