HTC One User Manual page 118

118 Phone calls
HTC One dials as is and does not prepend your home country
code when you call phone numbers that are not stored in HTC
One. When manually entering a phone number to call, you need to
enter a plus (+) sign and country code before the number.
Editing a country's international call settings
1. Go to Settings, and then tap Call.
2. Tap Assisted Dialing.
3. Under Reference Country, select the country whose settings you
want to edit.
4. Tap > Edit.
5. Edit the Country Code, IDD Prefix (International Direct Dial), NDD
Prefix (National Direct Dial), Area/City Code and National Number
Length (length of phone number).
6. Tap Save.
7. Tap Done.
Resetting the reference country to default
1. Go to Settings, and then tap Call.
2. Tap Assisted Dialing.
3. Under Reference Country, select the country whose settings you
want to set to default values.