Anybody can call 112 from an ordinary telephone (landline) or from a mobile phone (GSM) where there is coverage.
You can be put through to 112 even if the phone you are using is locked or has no SIM card.
You should call 112 only in the event of an emergency.
If you are not sure whether the situation at hand is an emergency, it is better to call than not to.
Information on open shelters in Reykjavik.