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The obvious way to read a number aloud is to read it as though it had been spelled out in words rather than given as digits. So 245 is read aloud as doscientos cuarenta y cinco.
What about numbers which are just identifiers like Boeing 737, hotel rooms, numbers in catalogues like Bach's cantata Wachet auf BWV 140. Is it optional whether to read them as a number or to spell out the digits? So would I say Boeing siete tres siete
Yes, you do have choices.
As though it were quantifying something: doscientos cuarenta y cinco (even though it isn't).
Digit by digit: dos cuatro cinco.
Combination: dos cuarenta y cinco.
If there's no quantified meaning, the most common approach is to use whatever will be succinct (small number of syllables), hence siete-tres-siete vs. Uno-cuarenta.
Bonus information: if you want to talk about a bus number (bus route), here is what is customary in Mexico at least:
Voy a tomar la Ruta Uno.
