The fixed dialing is handset+SIM feature, that does not depend of the operator. All you need is the PIN2 (usually is not the same as PIN).
When you set the FDN numbers, just enter the prefix you want, and it will allow you to call only numbers that start with that prefix. FDN only checks if the begining of the number you want to dial matches what was set as fixed dialing number.
If you want to allow calls only to one number, you enter the full number (with or without country code - doesn't matter):
0531122334455 (I assume 053 as the prefix of the operator). You will only be allowed to dial this number.
If you want to allow calls only to same operator, you put the prefix of the operator:
You will be allowed to dial any number starting with 053 (like 053001122334455, 0539988776655, etc.). You will not be able to dial i.e. 054xxxxx numbers.