After visit abroad phone still dialling international STD code?

iPhones have a feature called Dial Assist that automatically adds the appropriate country code when you dial a number.This can be helpful, but it might cause issues if it doesn't recognize you're back in your home country.

Go to Settings > General > Date & Time. Turn on any of the following: Set Automatically: iPhone gets the correct time over the network and updates it for the time zone you're in. Some networks don't support network time, so in some countries or regions, iPhone may not be able to automatically determine the local time.

Change the date and time on iPhone - Apple Support