To dial any UK telephone number from Italy (or most of Europe), replace the trunk prefix 0 of the UK number with telephone country code +44. If you're on a mobile phone, enter the number beginning with the plus symbol; for example +44 20 7946 0123. If you're on a landline, use 00 44 for +44.
Note that, for calls in the opposite direction, you do not drop the trunk prefix 0 from the Italian number. For example, a number in Rome 06 becomes +39 06 or 00 39 06.
(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)
+43
011 is the international calling code from North America. 44 is the country code for the United Kindom. 20 (or 020 when dialling within the UK) is the area code for London. So, a number like 011 44 20 3890 1234 would be a telephone number in London, UK.
Qatar is telephone country code +974.
No. London numbers start with code 020 (just 20 if calling from overseas) followed by an eight digit number starting with 3, 7 or 8, so a London number would look like: 020 3456 7890
This isn't a London code. It is the area code for Brentwood in Essex which is about 30 miles northeast of London.
00 44 20 More generally, replace the leading 0 of the UK domestic number with 00 44.
That code is not standardized; it depends on your telephone company.
020 is the telephone dialling code for all of London.
To call Germany from another country, dial +49 then the German area code and telephone number. Replace the + with 00 if calling from Europe, 011 if calling from US or Canada, etc.
New York City (excluding Manhattan)
London's telephone code is 020 followed by an eight digit number which will start with 3,7 or 8. The number that you have asked about is not a London number as you have described it.
001 and then the New York number including the area code