Country code does not appear on caller ID when roaming

Hi

I am not sure if anyone of you have encountered this problem. I have an unlocked iPhone from my service provider in Singapore. When I am in Singapore, all my incoming calls will be displayed with the correct country code i.e +65 xxxx xxxx if it is from Singapore and +1 (xxx) xxxx xxxx if the call is from the USA. I noticed that when I am roaming in the USA, incoming calls e.g. from Singapore will not have the country code e.g. +65 1234 5678 is displayed as +12 3456 78 and I am unable to answer the call as clicking on "Answer" will terminate the call. I also noticed that calls from the USA are displayed correctly with the +1 (xxx) xxxx xxxx format.

Do you have any advice, please?

Thanks n kind regards

Stan