Mobile prefixes are either two or three digits, between the range of 602 to 608 and 72 to 79. The total number of digits should be 9. Numbers can be kept if you change providers. If you call a mobile number that is no longer registered with the original provider, a short message will tell you that the call will cost more.
Oftentimes, there is a free contract for Czech citizens and non-Czech citizens need a residency permit and then pay up to 2,000 CZK deposit (for long-term residency). The deposit will be returned when the contract is terminated if there is no outstanding debt.
Telefonica O2
(was Eurotel) Recently started a massive advertising campaign.
Address: Praha 1, Vaclavske namesti 813/57
Open: 09:00-20:00, Sat 09:00-18:00, Sun 11:00-18:00.
Vodafone
(was Oskar) This is the cheapest option, but has the worst coverage.
Address: Praha 1, Vaclavske nam. 47
Open: 09:00-21:00, Sat 09:00-18:00, Sun 10:00-18:00.
T-Mobile
Address: Praha 1, Na Prikope 23
Open 09:00-20:00, Sat 09:00-18:00, Sun 10:00-18:00.
For short term phone use, pay as you go is a great option. There is an inflated per-minute rate compared to contract, but it is easy to cancel at any time.
It is also possible to rent mobile phones directly from the Prague airport. The convenience is excellent, but prices are high. Check at the information desk for prices.