Whether or not your phone will work abroad depends on a number of things: your phone, your network, and the country to which you are travelling. It is important to bear in mind that while some phones will work in some countries they may not work in others. For your phone to work in the country you are visiting it will need to be compatible with the type of network employed there, and your mobile phone operator will need to have an agreement in place with a local operator.
Firstly, not all phones connect in the same way and there are different network protocols in use throughout the world. The predominant protocol is GSM which is used by roughly 80% of the world´s mobile phone networks and is the main system operated in Europe, Asia, Africa and Brazil.
For most of North and South America a different system is used that is known as CDMA. Because CDMA and GSM are based on conflicting technological principles GSM phones will not work on a CDMA network, and vice versa. As the system used by all network operators in the UK is GSM this means that British phones will not work on American CDMA networks. Even so, while CDMA is the predominant network protocol in the United States there are a number of phone networks, such as T-Mobile USA and AT&T, which do operate GSM networks.
As GSM is the predominant technology globally you stand a better chance travelling abroad with a GSM phone than you would with a CDMA phone. While the country you are visiting may have GSM networks in place there are still more factors to consider. There are four frequencies at which GSM signals are transmitted: 850MHz, 900MHz, 1800MHz and 1900MHz. Most of North and South America use 850MHz and 1900MHz frequencies for GSM networks, while the rest of the world uses 900MHz and 1800MHz frequencies.
A British phone that only works on 900MHz and 1800MHz bands will be non-functional on US networks employing GSM. In order to be able to use a GSM phone on any GSM network in the world it needs to be tri-band or quad-band GSM enabled, meaning that it can operate on 3 or 4 GSM frequencies respectively. Most GSM phones now fall into this category allowing them to be used on most GSM networks throughout the world. However, many phones are restricted to the GSM frequencies used in their target markets so double check the GSM capabilities of your phone and the GSM frequencies in use in your chosen destination before you depart to ensure that you can use your phone while abroad.
This is not all that needs to be considered however. Even though your phone may be quad-band GSM and you are travelling to a country that has a GSM network in place there is one more factor that needs to be taken into account. As your phone operates through a UK based network their services will not be available in the country you are visiting and you will need to connect through a local network operator. In order to do this your network provider will need to have roaming agreements in place with local network operators for the country you are visiting.
The agreements in place with local networks will differ in what services they offer, with some countries allowing a full range of features like video calling while in other countries you may be limited to sending and receiving SMS messages only, and in other countries there may be no agreements in place for your network at all. The larger networks have agreements in place in many countries, but if you are unsure you can check your network provider´s homepage to see whether they have agreements in place for the country you are travelling to.
If your network provider does not have an agreement in place with a local operator in the country that you are travelling to then you may still be able to use your phone by purchasing a SIM card on arrival in that country. This may limit you to making calls in that country however, as phone calls back to the UK will be charged at (expensive) international rates. To be able to use a local SIM card you will need to have an unlocked phone that is not tied to a particular network. Alternatively there are services that offer global roaming SIM cards that can be used freely across the globe and can be purchased before you start your travels. These may be a good option to go for if you intent to travel a lot from country to country as they can offer a consistently cheaper rate than standard roaming tariffs.