Note that to actually call any non-US number you will need to first enable international calling on your phone through Verizon.
I called Verizon's global assist customer service (or whatever they call themselves) and NO ONE told me that. In fact the agent emailed me this while we were on the phone:
About Your Apple iPhone SE
Your device will work in England and Gibraltar.
However, you must check the device settings for use outside the U.S.
Go to Settings > Cellular (Cellular Data must be On) > Cellular Data Options > Roaming
Switch Voice Roaming On (green) and Data Roaming On (green). Voice or Data can be turned Off (white) if needed.
Turn International CDMA Off (white).
Power the device off and then back on after any changes OR when entering a country/cruise ship.
Messaging notes
If sending messages to other Apple devices and iMessage is enabled, messages are billed as data.
If you receive Message Delivery Failed errors, enable Send as SMS in Message settings.
More info
To turn off ALL cellular network service, switch Airplane Mode On (green) in Settings. You can still use Wi-Fi with this option.
If you use Wi-Fi, we recommend disabling Wi-Fi Assist in Settings > Cellular. Scroll down to see this option. This applies to iOS 9 and above.
TravelPass: use your domestic plan allowances for a daily fee
Per line/day Available Countries Allowance
$10 England and Gibraltar Uses Domestic Talk, Message, and Data
A 24 hour session starts when you place or answer a call, send a text or use data in a TravelPass country.
This includes background data and automatic updates. Incoming texts do NOT start a session.
If you don’t use service, you won't be charged the daily fee.
When ½ GB of data is used in one TravelPass session, the data speed will be reduced. Speed will resume with the next session.
No need to remove TravelPass when you return. It doesn't bill in the US.
Calls to numbers within the TravelPass country and calls to US numbers are covered by your domestic plan.
Calls to numbers of other countries are charged International Long Distance.
No mention of enabling international calling.