I have no references for this, I'm merely a native Swedish speaker with linguistics as a keen armature interest, but I have a hypothesis:
Whatever is going on, past tense seems to be used in a similar way in other languages, such as English or Spanish. I think the past tense could be considered a "polite verb form", though it may (have evolved to?) be used in other-than-polite ways as well.
In English, I would interpret the following as a polite present tense, though of course they could be interpreted literally as well.
- Could you show me the way?
- I was wondering if you could pass some potatoes.
- I wanted to ask you for help.
Could and would are probably the most common words used in this context. (If someone objects that they are really conditional, not past tense, my point still holds: People use verb forms outside their literal meaning for politeness)
I know similar constructs in Spanish, and I think I remember there's something similar in German, though I'm less certain.
In Swedish, the following makes sense in a present-tense context:
- Jag hade en fråga. (I had a question)
- Ville du ha något? (Did you want something?)
- (archaic) Ville Fröken ha något? (Did the Miss (you) want something?)
- Namnet var Karl. (My name is Karl, lit. The name was Karl)
I include the archaic form addressing someone with Fröken for the sake of possible Swedish readers, as I find that trying to talk like an old movie make these past-tense constructions show up all the time.
I'm not sure they are necessarily more polite than they would be in present tense, but polite language and politeness don't always go hand in hand. (Compare the, admittedly extreme, English example "Would someone please put the bastard out of his fucking misery?". Despite all the polite forms and the euphemism, it's quite rude.)
Whatever is going on, it's not limited to answering the phone. It's not unique to Swedish but happens in English and other languages as well.