Slovakia uses international prefix 421 (+421).
Numbers have standard format with 9 digits.
There are three mobile operators in Slovakia:
They all operate GSM standard 2G networks with wide coverage of the country, complemented by 3G and LTE coverage in urban areas.
If you want to avoid expensive roaming charges when in Slovakia, you can purchase a prepaid SIM card. Please note that your passport or ID card can be required when buying a SIM card (you don't need it when buying coupons to top your credit) and the operator is entitled to scan or copy the passport or ID card according to Slovak telecommunications legislation.
Probably the most suitable prepaid card for tourists is called Tesco Mobile (which uses services of O2 networks) and it can be purchased at Tesco supermarkets.The card can be used also for data communication (access to the internet from the phone).