If they call you with an offer, and if they ask you, would you like to accept, you can respond something similar to this:

"Yes I accept pending my reading of the offer letter"

Never accept a job without reading the offer letter first. The offer letter will typically come with a statement, indicating how many days you have before you need to return the letter to the employer. Take that time to decide.

Normally, if they ask me to sign on the spot, I will decline the request. Unless that is a job I really want.

But so far I have not encountered any such company..