As a rule, Core Lite applies the same country code of your home mobile phone subscriber plan i.e. for US residents it is +1, to your contacts who do not already have a country code prefix applied.
This ensures that if you travel outside the US, your contacts will still receive SMS from you when using sharing features.
If the country code prefix of a contact needs to be updated, and they reside outside outside your home jurisdiction, apply the country code prefix for their country.
You can view the status of contacts in the contacts card in your mobile phone or when selecting contacts for notifications in a safety plan.