In order to customize the phone number input field country flags that appear on the left side of the input, you have to add the ‘whitelisted’ country codes each per line (no commas, dots etc.).
However, this does not restrict or allow users to input a phone number from a different country and receive calls. This functionality is strictly managed by Twilio and you should check your zone restriction following this link.
Here is the full supported Country List and their corresponding 2 letters code:
Afghanistan (افغانستان) => af
Albania (Shqipëri) => al
Algeria (الجزائر) => dz
American Samoa => as
Andorra => ad6
Angola => ao
Anguilla => ai
Antigua and Barbuda => ag
Argentina => ar
Armenia (Հայաստան) => am
Aruba => aw
Australia => au
Austria (Österreich) => at
Azerbaijan (Azərbaycan) => az
Bahamas => bs
Bahrain (البحرين) => bh
Bangladesh (বাংলাদেশ) => bd
Barbados => bb
Belarus (Беларусь) => by
Belgium (België) => be
Belize => bz
Benin (Bénin) => bj
Bermuda => bm
Bhutan (འབྲུག) => bt
Bolivia => bo
Bosnia and Herzegovina (Босна и Херцеговина) => ba
Botswana => bw
Brazil (Brasil) => br
British Indian Ocean Territory => io
British Virgin Islands => vg
Brunei => bn
Bulgaria (България) => bg
Burkina Faso => bf
Burundi (Uburundi) => bi
Cambodia (កម្ពុជា) => kh
Cameroon (Cameroun) => cm
Canada => ca
Cape Verde (Kabu Verdi) => cv
Caribbean Netherlands => bq
Cayman Islands => ky
Central African Republic (République centrafricaine) => cf
Chad (Tchad) => td
Chile => cl
China (中国) => cn
Colombia => co
Comoros (جزر القمر) => km
Congo (DRC) (Jamhuri ya Kidemokrasia ya Kongo) => cd
Congo (Republic) (Congo-Brazzaville) => cg
Cook Islands => ck
Costa Rica => cr
Côte d’Ivoire => ci
Croatia (Hrvatska) => hr
Cuba => cu
Curaçao => cw
Cyprus (Κύπρος) => cy
Czech Republic (Česká republika) => cz
Denmark (Danmark) => dk
Djibouti => dj
Dominica => dm
Dominican Republic (República Dominicana) => do
Ecuador => ec
Egypt (مصر) => eg
El Salvador => sv
Equatorial Guinea (Guinea Ecuatorial) => gq
Eritrea => er
Estonia (Eesti) => ee
Ethiopia => et
Falkland Islands (Islas Malvinas) => fk0
Faroe Islands (Føroyar) => fo
Fiji => fj
Finland (Suomi) => fi
France => fr
French Guiana (Guyane française) => gf
French Polynesia (Polynésie française) => pf
Gabon => ga
Gambia => gm
Georgia (საქართველო) => ge
Germany (Deutschland) => de
Ghana (Gaana) => gh
Gibraltar => gi
Greece (Ελλάδα) => gr
Greenland (Kalaallit Nunaat) => gl
Grenada => gd
Guadeloupe => gp
Guam => gu
Guatemala => gt
Guinea (Guinée) => gn
Guinea-Bissau (Guiné Bissau) => gw
Guyana => gy
Haiti => ht
Honduras => hn
Hong Kong (香港) => hk
Hungary (Magyarország) => hu
Iceland (Ísland) => is
India (भारत) => in
Indonesia => id
Iran (ایران) => ir
Iraq (العراق) => iq
Ireland => ie
Israel (ישראל) => il
Italy (Italia) => it
Jamaica => jm
Japan (日本) => jp
Jordan (الأردن) => jo
Kazakhstan (Казахстан) => kz
Kenya => ke
Kiribati => ki
Kuwait (الكويت) => kw
Kyrgyzstan (Кыргызстан) => kg
Laos (ລາວ) => la
Latvia (Latvija) => lv
Lebanon (لبنان) => lb
Lesotho => ls
Liberia => lr
Libya (ليبيا) => ly
Liechtenstein => li
Lithuania (Lietuva) => lt
Luxembourg => lu
Macau (澳門) => mo
Macedonia (FYROM) (Македонија) => mk
Madagascar (Madagasikara) => mg
Malawi => mw
Malaysia => my
Maldives => mv
Mali => ml
Malta => mt
Marshall Islands => mh
Martinique => mq
Mauritania (موريتانيا) => mr
Mauritius (Moris) => mu
Mexico (México) => mx
Micronesia => fm
Moldova (Republica Moldova) => md
Monaco => mc
Mongolia (Монгол) => mn
Montenegro (Crna Gora) => me
Montserrat => ms
Morocco (المغرب) => ma
Mozambique (Moçambique) => mz
Myanmar (Burma) (မြန်မာ) => mm
Namibia (Namibië) => na
Nauru => nr
Nepal (नेपाल) => np
Netherlands (Nederland) => nl
New Caledonia (Nouvelle-Calédonie) => nc
New Zealand => nz
Nicaragua => ni
Niger (Nijar) => ne
Nigeria => ng
Niue => nu
Norfolk Island => nf
North Korea (조선 민주주의 인민 공화국) => kp0
Northern Mariana Islands => mp0
Norway (Norge) => no
Oman (عُمان) => om
Pakistan (پاکستان) => pk
Palau => pw
Palestine (فلسطين) => ps
Panama (Panamá) => pa
Papua New Guinea => pg
Paraguay => py
Peru (Perú) => pe
Philippines => ph
Poland (Polska) => pl
Portugal => pt
Puerto Rico => pr
Qatar (قطر) => qa
Réunion (La Réunion) => re
Romania (România) => ro
Russia (Россия) => ru
Rwanda => rw
Saint Barthélemy (Saint-Barthélemy) => bl
Saint Helena => sh
Saint Kitts and Nevis => kn
Saint Lucia => lc
Saint Martin (Saint-Martin (partie française)) => mf
Saint Pierre and Miquelon (Saint-Pierre-et-Miquelon) => pm
Saint Vincent and the Grenadines => vc
Samoa => ws
San Marino => sm
São Tomé and Príncipe (São Tomé e Príncipe) => st
Saudi Arabia (المملكة العربية السعودية) => sa
Senegal (Sénégal) => sn
Serbia (Србија) => rs
Seychelles => sc
Sierra Leone => sl
Singapore => sg
Sint Maarten => sx
Slovakia (Slovensko) => sk
Slovenia (Slovenija) => si
Solomon Islands => sb
Somalia (Soomaaliya) => so
South Africa => za
South Korea (대한민국) => kr
South Sudan (جنوب السودان) => ss
Spain (España) => es
Sri Lanka (ශ්රී ලංකාව) => lk
Sudan (السودان) => sd
Suriname => sr
Swaziland => sz
Sweden (Sverige) => se
Switzerland (Schweiz) => ch
Syria (سوريا) => sy
Taiwan (台灣) => tw
Tajikistan => tj
Tanzania => tz
Thailand (ไทย) => th
Timor-Leste => tl0
Togo => tg
Tokelau => tk
Tonga => to
Trinidad and Tobago => tt
Tunisia (تونس) => tn
Turkey (Türkiye) => tr
Turkmenistan => tm
Turks and Caicos Islands => tc
Tuvalu => tv
U.S. Virgin Islands => vi
Uganda => ug
Ukraine (Україна) => ua
United Arab Emirates (الإمارات العربية المتحدة) => ae
United Kingdom => gb
United States => us
Uruguay => uy
Uzbekistan (Oʻzbekiston) => uz
Vanuatu => vu
Vatican City (Città del Vaticano) => va
Venezuela => ve
Vietnam (Việt Nam) => vn
Wallis and Futuna => wf
Yemen (اليمن) => ye
Zambia => zm
Zimbabwe => zw