The carrier for a number like +65 8391 1500 is probably StarHub
In NATIONAL format, aka ITU-T Recommendation E123, the number is written 8391 1500
Likewise, in E123 INTERNATIONAL formal, it is written +65 8391 1500
In E.164 standard format, the number is written +6583911500. E164 format is as per E123 INTERNATIONAL format but with no formatting applied.
In RFC3966 standard format, the number is written tel:+65-8391-1500. FC3966 is as per INTERNATIONAL format, but with all spaces and other separating symbols replaced with a hyphen, and with any phone number extension appended with ";ext=". It also will have a prefix of "tel:" added .
To dial this number from within the United States, one would dial 011 65 8391 1500
To dial this number from inside the United Kingdom or the European Union, one would dial 00 65 8391 1500
Numbers like +6583911500 may be located in the Asia/Singapore timezone, where the local time is currently Wednesday the 25th of November 2020 04:36:42 PM.