In NATIONAL format, aka ITU-T Recommendation E123, the number is written (081) 755-8648
Likewise, in E123 INTERNATIONAL formal, it is written +1 081-755-8648
In E.164 standard format, the number is written +10817558648. E164 format is as per E123 INTERNATIONAL format but with no formatting applied.
In RFC3966 standard format, the number is written tel:+1-081-755-8648. 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 1 (081) 755-8648
To dial this number from inside the United Kingdom or the European Union, one would dial 00 1 081-755-8648