How to add country phone number prefix in webform?
Webform allows you to load options in bulk. Just google for this list and load it into a spreadsheet. Use a bit of find/replace to get it into the format needed by a webform select element, which is:

65|Singapore (65)
44|United Kingdom (44)
58|Venezuela (58)


Then create a webform select field and paste it in.

To get the fields to line up side-by-side (with country code right before phone number and not on a separate line) use the webform_layout module.