A family of Microsoft word processing software products for creating web, email, and print documents.
You can do this with a series of conditional fields e.g. assuming the bookmark name of the dropdown field is Dropdown1 - check its calculate on exit check box property and at the place you want the number to appear create a series of conditional fields with no spaces between them - one for each entry in your dropdown:
{ IF { REF Dropdown1 \*lower } = "sydney" "123 4567"}{ IF { REF Dropdown1 \*lower } = "london" "456 1234"}{ IF { REF Dropdown1 \*lower } = "new york" "999 7777"}
Then as you tab out of the dropdown field, (with the form protected for forms) the appropriate number will show. If you don't use the \*lower switches, you will have to use the exact case as shown in the dropdown entries.