This is not a real bug, but a workaround. I explain you how it works.
The numbers "0039340xxxx", "+39340xxxx" and "340xxxx" are the same number, but the first two have the national prefix in two different way. Therefore, the phone couldn't make an exact comparison of the phone, because if your phone book contact has the number "340xxxx", but when you receive a call/sms the operator send you the number "+39340xxxx" the phone doesn't recognize it, because "340xxxx" is different from "+39340xxxx".
The fastest solution (workaround) to fix this issue is to choose a number of digits big enough to recognize the number uniquely in the 99% of case as the probability that the same user has two almost equals number (different in the first few digits) is really really low.

Then, their solution is quite good, but not perfect and you are really unlucky to be in that 1% not covered by the workaround