How do you deal with students turning on technology or cellphones in the classroom?

This is a very frustrating situation for a teacher. First, you should have a clear technology policy in your syllabus. You should make sure that the policy conforms to general school and departmental policy guidelines and make explicit exceptions for cases where the technology is part of an accommodation for a documented disability. 

Some instructors collect cellphones at the beginning of a class period and return them at the end, but this could lead to...

This is a very frustrating situation for a teacher. First, you should have a clear technology policy in your syllabus. You should make sure that the policy conforms to general school and departmental policy guidelines and make explicit exceptions for cases where the technology is part of an accommodation for a documented disability. 

Some instructors collect cellphones at the beginning of a class period and return them at the end, but this could lead to legal liability for the cost of the cell phone if one were lost or stolen, and thus is imprudent.

Part of how you handle technology use in the classroom depends on the size of the classroom. In large lecture classes, one technique is to divide the classroom with the front reserved for a "no technology" section and the back for technology. This means that the students who are spending the class period playing video games or using social media end up in the back of the room, where they do not distract the students actually trying to learn the material. Occasionally you need to move a technology-using student to the back of the classroom, but in general, the slackers prefer sitting at the back anyway.

Another possibility is walking around the classroom as you teach and, if you see a student using technology inappropriately, simply standing near the student and explaining that the class will continue when the offending device has been turned off and put away. If your school policy allows you to do so, you should ask repeat offenders to leave the classroom. 

Some teachers try to incorporate technology into their classes, but that can be problematic, as once students have tablets or cell phones out and turned on, the temptation to check social media and text messages or play games is usually irresistible.