HTC One Remix - Manual (Page 191)

How do I add an extension number to a
contact number?
While creating a new contact in People, you can add an extension
number to their number so you can skip the voice prompts when
calling.
1. After entering the number of the main line, do one of the
following:
§ Tap P(,) to insert a pause before the extension number
is dialed automatically. To make the pause longer, tap
P(,) again.
§ Tap W(;) to be prompted to confirm the extension
number.
2. Enter the extension number.
People 191