What is Dial Assist? Dial Assist on iPhone automatically adds international and local prefixes to phone numbers, so you don't have to enter them yourself. This feature is designed to be helpful, particularly when calling internationally since the call won't go through without the correct prefix. But for people who travel often, it may be preferable to save full international numbers with prefixes in their contacts, making Dial Assist redundant. If you'd like to turn off Dial Assist on your iPhone, read on.