What is Dial Assist? Dial Assist on the 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.
In short, if you happen to be from the USA and you travel abroad.
Your iPhone determines your Time Zone (If set to Automatic)
Your iPhone knows the region as USA (Settings > General > Language and Region)
When You dial from your current Time Zone using any number on the Contact list, +1 is prefixed to the number of all non-prefixed numbers
If you had stored +44 2345 678901 (Dial Assist has no role to play)
If you had stored 234567890 for a contact in the USA then Dial assist will prefix +1 if you are travelling outside of the USA.
If you had stored 234567890 for a contact in the UK even then Dial assist will prefix +1 if you are travelling outside of the USA.
It is always recommended that you store correct prefix for international contacts by adding "+" International Code.
