The "scrum master" is not telling you how long a ticket will take. They're asking you, can you break this ticket down any further?
I see that you have a task to implement validation in the phone number field, can we break that down into three tasks? Determining the format which needs to be applied and edge cases, adding validation to the field on the client side, and also on the server side?
Right away, thinking about it this way may reveal certain unknowns which you had not previously thought about. And all of it took an extra minute or two. While your team-mates are also engaged and "THERE" to help.
Later you can see which of the tasks took longer or less than expected, and use this to improve your future estimates (during the retro.)
This is only skimming the top of the types of benefits you can gain from a good Agile process.
