Do you have a fixed line at work? Why not let your employer pay for your international calls? Calling 81510010 will cost 59 øre/min if you have Telio and 99 øre/min if you have a Telenor subscription.
As you know many big companies like banks and insurance companies use a 5 digit number for their support phone or they use a number starting with 815xxxxx. It means it costs the same for your employer if you call your bank in Norway, or if you call your friend in another country (via 81510010).
So if it is okay for your employer to pay the cost for 81510010, you will be able to call almost everywhere for free! If you have a mobile phone from your work, do not use it via 81510010, it will be quite expensive via mobile phones. You can use 46002205 from mobile phones.
If you are an employer, you should also concider to block all international calls and ask your employees to call international numbers via 81510010. In this way you can control the costs. Usually it is much more expensive to call international numbers directly than via 81510010.