Ensure roaming is on for you.
I think the rule is that service suppliers aren't allowed to charge extra to recoup the additional costs of international roaming any more. I assume that includes Norway but check with your supplier. Worst comes to worst you can buy a local Norwegian SIM.
No, it should all happen automatically. I always receive a text from O2, welcoming me to France/Germany/Spain and telling me that calls, texts etc will come out of my usual monthly allowance.
As has been indicated, Norway is a member of the EEA and thus (under EU laws) roaming charges are prohibited for visitors who're using a phone registered in any other EEA country (such as the UK).
As has also been indicated, the obligation for UK phone providers not to apply roaming charges will cease to apply after the UK leaves the EU, so it's possible that we might return to the position where it will cost you megabucks to use your phone abroad.
'it's possible that we might return to the position where it will cost you megabucks to use your phone abroad.'
But ......All EU rules and regulations, including on mobile roaming anywhere in the EU, will continue to apply until the end of the transition period on 31 December 2020, which may be extended for one or two years.
Just as a warning and I don't fully understand it but when I'm in Spain I can text and phone back to the UK at no extra charge from my Vodafone package but all texts and calls to Spanish numbers I was charged extra.