Mobiles
The mobile communications market has developed rapidly in Russia in the last few years, with 4G technology, better coverage, and increasingly low call costs among the improvements. If you are staying in the city for more than a few days, it will almost certainly be worth buying a local SIM-card (and a compatible phone if required).
To buy a SIM-card, you will need to go to a mobile phone shop with your passport. Nearly all plans in Russia are pre-paid, and there's a large but easily comprehensible range of choices depending on what you plan to do (the language barrier may make this more complicated, of course, but most networks offer a base plan with various add-ons that you can order via SMS). SIM-cards cost as little as $4, and in most cases that is converted directly into talk time. Be aware that Russian networks operate by region, so the rates advertised will only apply if you are using your phone in St. Petersburg. As soon as you leave the city, you will have to pay higher roaming rates to use your phone.
To top up the credit on your phone, you can either visit a mobile phone store (the advantage being that there will be no commission), or use one of the automatic pay-points located in nearly all grocery stores, metro stations, etc. These machines are very simple to use, and commission is usually around 8%.
Mobile modems can also be a great economy if you have to pay high costs for Wi-Fi at your hotel, or a simple solution for those who don't have Internet coverage at their accommodation or need access on the go.
Most mobile phones have what is known as federal numbers. These are normally ten digits long. The first digit is either "8" if you are calling from a landline, or "+7" from another mobile. Some more expensive tariffs have a seven-digit city number which can be called free from landlines. If you need to send an SMS to a phone with a city number, you will need to add "+7812" to the seven-digit subscriber number.