Local phone numbers you purchase work anywhere in the world. you can call your Local phone number from any phone and on any network. Local phone numbers are real phone numbers, just like your fixed landline phone number.
If you have a small business or you only use landlines and mobile phones on traditional networks, Local Phone Numbers are your best option.
Extension numbers only work with VoIP, VoIP to VoIP. You can't use a normal phone to call an Extension Number.
Extension Numbers only work with VoIP. For example, any VoIP user can call your Extension Number. You can forward calls from your Extension Number to any landline, mobile or sip phone.