How thorough are the employment Comcast background checks?

I just got offered a work at home position with Comcast... I just submitted my background information and was wondering how thorough are thier investigations? I am not worried about criminal offenses. I have never gotten into any criminal trouble. I graduated high school om time and have some college expirience. I am worried when it comes to my employment history though. Because it is very shakey. have always been hoping from one job to the next because when i got out of high school i was a mess and could barely remember the dates and titles of all. I put down the ones that i could remember. I always did some online work but i did not list those because that was not continous work. I did do a solid 9 months in a call center working full time but it was a very unstable work enviroment. I worked off and on there so i out down the duration of the time i worked there which was between 2 years. and i worked gamestop for 3 months but left because there werent many hours. They didnt ask for the phone numbers on the form so I m wondering do they actually call these people and talk with them? I have nothig to hide about my work ethic im just worried they might see something on my history that i forgot to mention and they link im lying. Please help I am very worried i may not get the job because of this :(.