Learn English – Which is the best way to start the conversation when we are calling a telephone number back

The suggestions in comments are good, but I tend to identify myself so they don't have to ask (especially if you think it might be a job opportunity!) I'd go with something like this:

Hi, this is [your name]. I have a missed call from this number, and I just wanted to call you back and see what you were calling about.

This is a bit wordier and more informal than some other suggestions, but I tried to go with what I think I would actually say on the phone, not necessarily what was the most concise/formal. If you want to remember the clearest, simplest response (above is just me winging it, I don't have a plan of what exactly I'm going to say before I call back) then I'd go with this:

Hi, this is [your name]. I received a missed call from this number; may I ask what your call was regarding?

Note that I still introduce myself; I think this is very important. They can't possibly answer your question ("What did you call me about?") until you've provided this information; they have to know who you are to know why they called!