Phone Number remove "Select Country Code"

Hi,

I'm trying to understand Bookings more instead of using something as an existing template. I have one booking , but I'm seeing two different things in regard to the phone number for two different services.

I noticed that when booking a service (figure 1a), it does not ask for a country code. When setting this service up, the phone number is mandatory (figure 1b). In another service, there is a drop down for the Country Code (figure 2) and the phone number is mandatory (figure 2b). How can I turn on/off the "Select Country Code" drop down? 

Figure 1a

Figure 1b

Figure 2a

Figure 2b

Thanks!