How to ask for time to consider a job offer/ answer deadline?

I've been applying to jobs nonstop and have been getting fairly regular interviews.

If I get an offer for one, how do I respond over the phone that I would like some time to think about it without sounding like I don't want the job?

And how common is it for people to accept a position knowing they won't be starting for a couple of weeks and then accept an offer for another job afterwards? Of course I don't want to do this, but I also want to do what is best for myself.

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