Why not: they had previously called?
That sentence is just slightly awkward in English.
Could this also be "They had rung earlier?"
Yes it could. Both on the phone and with a bell
They had called before??
Can I use før instead of tidligere here?
Yes you can
Why is this not "ringet til"? I thought these words always went together.
An example in the same module: I had called his father > jeg havde ringet til hans far.