"直通专用电传" can be translated as "dedicated direct-line telex" or "special direct telex service".
"Dedicated" or "special" emphasizes the exclusivity or specific purpose of the telex service.
"Direct-line" or "direct" indicates that the telex connection is a straight, uninterrupted link.
"Telex" is the term for a teleprinter exchange system used for transmitting written messages electronically.
Depending on the context and the level of formality, you might choose one translation over the other. "Dedicated direct-line telex" might be more precise in a technical or business setting, while "special direct telex service" could be more user-friendly in a general context.