TU (International Telecommunication Union) adalah sebuah badan dunia yang berkedudukan di Jenewa (Swiss) yang membuat standar informasi dan komunikasi dunia. Salah satu prosedur yang dikeluarkan yaitu rekomendasi E.164 (Mei 1997) mengenai hubungan telekomunikasi antar negara via telepon menggunakan prefix IDD (International Direct Dialling). Prefix IDD di Indonesia lebih dikenal dengan kode SLI (sambungan Langsung Internasional). Untuk melakukan hubungan tersebut kita hanya cukup menekan :
kode operator SLI + kode SLI + telepon tujuan
sebagai contoh untuk telepon ke Afghanistan dari Indonesia : 008 93 21456798
Sejak keterbukaan informasi, kini tersedia banyak operator yang melayani fasilitas SLI. Selain itu, berkembangnya teknologi internet juga telah melahirkan VoIP (Voice Over IP), yaitu melakukan komunikasi suara melalui jalur data. VoIP juga telah membuka peluang usaha dengan keluarnya kebijakan pemerintah tentang penyelenggaraan Jaringan Tetap Internasional. Dengan VoIP, komunikasi antar negara menjadi lebih murah dan bisa dinikmati semua kalangan. Untuk kode operator SLI tersedia berbagai pilihan sesuai yang diberikan operator penyelenggaranya (001, 007, 008, 017, 01016, 01017, 01010, hingga 010XY) dan bisa dibandingkan kualitas suara hingga tarif satu sama lainnya. Berikut kode SLI negara-negara tersebut. :
| No | Negara | Kode Negara |
| 1 | Afghanistan | 93 |
| 2 | Afrika Selatan | 27 |
| 3 | Afrika Tengah ,Rep | 236 |
| 4 | Albania | 355 |
| 5 | Algeria | 213 |
| 6 | Amerika Serikat | 1 |
| 7 | Andorra | 376 |
| 8 | Angola | 244 |
| 9 | Anguilla | 1-264 |
| 10 | Antigua dan Barbuda | 1-268 |
| 11 | Antilles Belanda | 599 |
| 12 | Argentina | 54 |
| 13 | Armenia | 374 |
| 14 | Aruba | 297 |
| 15 | Ascension | 247 |
| 16 | Australia | 61 |
| 17 | Austria | 43 |
| 18 | Azerbaijan | 994 |
| 19 | Bahamas | 1-242 |
| 20 | Bahrain | 973 |
| 21 | Bangladesh | 880 |
| 22 | Barbados | 1-246 |
| 23 | Belanda | 31 |
| 24 | Belarus | 375 |
| 25 | Belgia | 32 |
| 26 | Belize | 501 |
| 27 | Benin, Rep | 229 |
| 28 | Bermuda | 1-441 |
| 29 | Bhutan | 975 |
| 30 | Bolivia | 591 |
| 31 | Bosnia and Herzegovina | 387 |
| 32 | Botswana | 267 |
| 33 | Brazil | 55 |
| 34 | British Virgin Kep. | 1-284 |
| 35 | Brunei Darussalam | 673 |
| 36 | Bulgaria | 359 |
| 37 | Burkina Faso | 226 |
| 38 | Burundi | 257 |
| 39 | Cape Verde Kep. | 238 |
| 40 | Cayman Kep. | 1-345 |
| 41 | Ceko Rep | 420 |
| 42 | Chad Republic | 235 |
| 43 | Chili | 56 |
| 44 | China | 86 |
| 45 | Christmas Kep. | 61-8 |
| 46 | Cook Kep. | 682 |
| 47 | Cyprus | 357 |
| 48 | Denmark | 45 |
| 49 | Diego Garcia | 246 |
| 50 | Djibouti | 253 |
| 51 | Dominika | 1-767 |
| 52 | Dominikan Republic | 1-809 / 1-829 |
| 53 | Ekuador | 593 |
| 54 | El Salvador | 503 |
| 55 | Equitorial Guinea | 240 |
| 56 | Eritrea | 291 |
| 57 | Eslandia | 354 |
| 58 | Estonia | 372 |
| 59 | Ethiopia | 251 |
| 60 | Falkland Kep. | 500 |
| 61 | Faroe Kep. | 298 |
| 62 | Fiji Kep. | 679 |
| 63 | Filipina | 63 |
| 64 | Finlandia | 358 |
| 65 | Gabon Rep | 241 |
| 66 | Gambia | 220 |
| 67 | Georgia | 995 |
| 68 | Ghana | 233 |
| 69 | Gibraltar | 350 |
| 70 | Greenland | 299 |
| 71 | Grenada and Carriacuou | 1-473 |
| 72 | Grenadin Kep. | 784 |
| 73 | Guadeloupe | 590 |
| 74 | Guam | 1-671 |
| 75 | Guatemala | 502 |
| 76 | Guinea, Bissau | 245 |
| 77 | Guinea, Rep | 224 |
| 78 | Guyana | 592 |
| 79 | Haiti | 509 |
| 80 | Honduras | 504 |
| 81 | Hong Kong | 852 |
| 82 | Hungaria | 36 |
| 83 | India | 91 |
| 84 | Indonesia | 62 |
| 85 | Inggris | 44 |
| 86 | Irak | 964 |
| 87 | Iran | 98 |
| 88 | Irlandia | 353 |
| 89 | Israel | 972 |
| 90 | Italia | 39 |
| 91 | Jamaika | 1-876 |
| 92 | Jepang | 81 |
| 93 | Jerman | 49 |
| 94 | Kamboja | 855 |
| 95 | Kamerun | 237 |
| 96 | Kanada | 1 |
| 97 | Kazakhstan | 7 |
| 98 | Kenya | 254 |
| 99 | Kiribati | 686 |
| 100 | Kolombia | 57 |
| 101 | Komoro | 269 |
| 102 | Kongo | 242 |
| 103 | Korea Selatan | 82 |
| 104 | Korea Utara | 850 |
| 105 | Kosovo | 422 |
| 106 | Kosta Rica | 506 |
| 107 | Kroasia | 385 |
| 108 | Kuba | 53 |
| 109 | Kuracao | 599 |
| 110 | Kuwait | 965 |
| 111 | Kyrgyzstan | 996 |
| 112 | Laos | 856 |
| 113 | Latvia | 371 |
| 114 | Lesotho | 266 |
| 115 | Libanon | 961 |
| 116 | Liberia | 231 |
| 117 | Libya | 218 |
| 118 | Liechtenstein | 423 |
| 119 | Lithuania | 370 |
| 120 | Luxembourg | 352 |
| 121 | Macao | 853 |
| 122 | Macedonia | 389 |
| 123 | Madagaskar | 261 |
| 124 | Maladewa | 960 |
| 125 | Malawi | 265 |
| 126 | Malaysia | 60 |
| 127 | Mali Rep | 223 |
| 128 | Malta | 356 |
| 129 | Mariana Kep. | 1-670 |
| 130 | Maroko | 212 |
| 131 | Marshall Kep. | 692 |
| 132 | Martinique | 596 |
| 133 | Mauritania | 222 |
| 134 | Mauritius | 230 |
| 135 | Mayotte Kep. | 269 |
| 136 | Mesir | 20 |
| 137 | Mexico | 52 |
| 138 | Micronesia, Fed States | 691 |
| 139 | Midway Kep. | 1-808 |
| 140 | Miquelon & St. Pierre | 508 |
| 141 | Moldova | 373 |
| 142 | Monako | 377 |
| 143 | Mongolia | 976 |
| 144 | Montenegro | 382 |
| 145 | Montserrat | 1-664 |
| 146 | Mozambik | 258 |
| 147 | Myanmar | 95 |
| 148 | Namibia | 264 |
| 149 | Nauru | 674 |
| 150 | Nepal | 977 |
| 151 | Nevis | 1-869 |
| 152 | New Caledonia | 687 |
| 153 | Niger Rep | 227 |
| 154 | Nigeria | 234 |
| 155 | Nikaragua | 505 |
| 156 | Niue | 683 |
| 157 | Norfolk Kep. | 61-672 |
| 158 | Norway | 47 |
| 159 | Oman | 968 |
| 160 | Pakistan | 92 |
| 161 | Palau | 680 |
| 162 | Palestina | 970 |
| 163 | Panama | 507 |
| 164 | Pantai Gading | 225 |
| 165 | Papua New Guinea | 675 |
| 166 | Paraguay | 595 |
| 167 | Peru | 51 |
| 168 | Polandia | 48 |
| 169 | Portugal | 351 |
| 170 | Prancis | 33 |
| 171 | Prancis Guiana | 594 |
| 172 | Prancis Polynesia | 689 |
| 173 | Principe | 239 |
| 174 | Puerto Rico | 1-787 / 1-939 |
| 175 | Qatar | 974 |
| 176 | Reunion Kep. | 262 |
| 177 | Romania | 40 |
| 178 | Rusia | 7 |
| 179 | Rwanda | 250 |
| 180 | Samoa | 685 |
| 181 | Samoa-Amerika | 1-684 |
| 182 | San Marino | 378 |
| 183 | Sao Tome | 239 |
| 184 | Saudi Arabia | 966 |
| 185 | Selandia Baru | 64 |
| 186 | Senegal Rep | 221 |
| 187 | Serbia, Rep | 381 |
| 188 | Seychelles | 248 |
| 189 | Sierra Leone | 232 |
| 190 | Singapura | 65 |
| 191 | Slovakia | 421 |
| 192 | Slovenia | 386 |
| 193 | Solomon Kep. | 677 |
| 194 | Somalia Rep | 252 |
| 195 | Spanyol | 34 |
| 196 | Sri Lanka | 94 |
| 197 | St. Helena | 290 |
| 198 | St. Kitts | 1-869 |
| 199 | St. Lucia | 1-758 |
| 200 | St. Vincent | 1-784 |
| 201 | Sudan | 249 |
| 202 | Suriname | 597 |
| 203 | Swaziland | 268 |
| 204 | Swedia | 46 |
| 205 | Swiss | 41 |
| 206 | Syria | 963 |
| 207 | Taiwan | 886 |
| 208 | Tajikistan | 992 |
| 209 | Tanzania | 255 |
| 210 | Thailand | 66 |
| 211 | Timor Leste | 670 |
| 212 | Togo | 228 |
| 213 | Tokelau | 690 |
| 214 | Tonga | 676 |
| 215 | Trinidad dan Tobago | 1-868 |
| 216 | Tunisia | 216 |
| 217 | Turki | 90 |
| 218 | Turkmenistan | 993 |
| 219 | Turks dan Caicos Kep. | 1-649 |
| 220 | Tuvalu | 688 |
| 221 | Uganda | 256 |
| 222 | Ukraina | 380 |
| 223 | Uni Emirat Arab | 971 |
| 224 | Uruguay | 598 |
| 225 | US Virgin Kep. | 1-340 |
| 226 | Uzbekistan | 998 |
| 227 | Vanuatu | 678 |
| 228 | Venezuela | 58 |
| 229 | Vietnam, Rep | 84 |
| 230 | Wake Kep. | 808 |
| 231 | Wallis and Futuna Kep. | 681 |
| 232 | Yaman | 967 |
| 233 | Yordania | 962 |
| 234 | Yunani | 30 |
| 235 | Zaire | 243 |
| 236 | Zambia | 260 |
| 237 | Zanzibar | 259-255 |
| 238 | Zimbabwe | 263 |