This varies by region.
In the US, the common way to write a telephone number is with the telephone area code (for example "206" is "Seattle") in brackets, and with a hyphen after the first triple:
My telephone number is (206) 123-4567
My telephone number is 206-123-4567
In the United Kingdom, the common way is to write a landline telephone number with a space after the area code (or the area code in brackets) and an additional space breaking up any seven remaining digits into a 3-4 tuple or eight remaining digits into a 4-4 tuple:
My telephone number is (01244) 123 4567
My telephone number is 01244 123 4567
My telephone number is 020 7738 1033
Mobile telephone numbers in the UK tend to be written "flat". In the US there is no distinction between mobile telephone numbers and landline ones, so the landline convention holds.
My telephone number is 07771234567
When writing international telephone numbers in the US, the format is usually thus:
My telephone number is +1 (206) 123-4567
In the UK when writing international telephone numbers, the "0" digit at the start of the area code is dropped for international dial-ins, so the leading zero is bracketted:
My telephone number is +44 (0)7771234567
My telephone number is +44 (0)20 7738 1033
However, if in doubt, just write the number direct without spaces or brackets. Be sure to include the international prefix if your telephone number is not from the same country as the person you are talking to. For instance, if you are currently living in the Netherlands and you are applying for a job in the United States, be sure to include the international dialing prefix for the Netherlands in your telephone number so that the person you are talking to can phone you.