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I'd recommend getting a EU Sim card primarily for data connectivity. That way you can book taxi online.
Most train or bus stations have free wifi. You can use that to book taxi online.
Uber is pretty reliable in Prague. Choose Uber pop for the cheapest Uber ride. If you know the time and place when you'll need a taxi schedule a Uber ride.
Other option is Modry Andel (ble anglel) - https://modryandel.cz/en
Hope that helps!