Write an article based on this "Bring a cell phone from the U.S. Buy a mobile phone in the UK. Call from a landline."
If you are from the United States and traveling in the United Kingdom, bring along an unlocked phone that allows SIM cards to be inputted from other providers. A SIM card is a small information chip that you can purchase from a mobile provider while abroad.  Ensure that the cell phone you want to bring to the UK is a GSM phone (global cellular technology used by most of the world) and it’s unlocked, meaning that it doesn't have a restriction on being used with other providers. AT&T and T-Mobile carry GSM phones which are typically unlocked.  Look for a UK mobile provider that carries a SIM card that will fit in your phone. Most mobile phones have a standard SIM card size, but iPhones and other newer smartphones use special micro or nano SIM card sizes.  You can set up an international calling plan through your current U.S. phone carrier, but note that these plans are typically very expensive. Whether you choose an international call plan or the SIM card route, make sure to call your U.S. provider to change or suspend your service and ask any questions you may have before traveling to the UK. Choose to buy a mobile phone in the UK if you can’t or don’t want to bring your own from the U.S., as it is frequently simpler and cheaper. Buy an inexpensive phone for a short stay in the UK, and purchase a pay-as-you-go SIM card plan to make international calls.  Check the pricing on the plan to make sure you know what the rates are for international calls. Check the data usage rates on a smartphone if you plan to use it for internet calling apps. Most pay-as-you-go SIM card plans for mobile phones in the UK come with a small amount of prepaid minutes for calls, and then you can “top up” with more minutes over the phone or at a kiosk for the network provider. The major mobile phone networks in the UK are Vodafone, T-Mobile, Virgin Mobile, O2, and Orange. Shops like Car Phone Warehouse and Phones4U sell inexpensive phones from several different networks. Call the U.S. from a UK landline phone if you wish. Check with the landline service provider to find out the rates for international calls, or purchase a calling card to lower the rates for calls to the U.S.  Purchase an international calling card from grocery stores, post offices, or phone shops in the UK to reduce the cost of frequent international calls. Generic international calling cards can usually be used with either landlines or mobile phones.  To use an international calling card, you typically will dial an access number found on the back of the card, then a unique PIN number, and finally the phone number you wish to dial.