Quantum key distribution is one of perspective directions of modern cryptography. It allows the parties of information exchange to develop a common cryptographic key, the secrecy of which is ensured by the laws of quantum mechanics. The paper studies the influence of the characteristics of the quantum channel, as well as the parameters of the applied algorithms on the maximum achievable length of a communication line for a protocol with phase-time coding and decoy states. By computational experiments with a simulation model of a quantum key distribution system based on the above protocol, it was found that the stable operation of the protocol is possible with line length of at most 210 km. It was also shown that this value can be increased by constructing more efficient sifted key correction algorithms.