First of all, from what I've heard declining a job offer after accepting is a looked upon as a huge indicator you are not suitable to work for any company. If you do that and word spreads, it could directly affect your career. Maybe if you absolutely have a dream job lined up with double the salary you should take that risk, but still think very very hard about it.
Second a phone interview IS NOT a job offer. I am a 4.0 GPA double major undergrad and 3.9 GPA masters student with great recommendations and tons of individual side projects and part time school-related job experience, but no full time job experience. So far I've been accepted one place, I turned it down due to a condition that would mean I could not work until half a year passed. I have had over a dozen interviews and half a dozen phone interviews. Most places go resume -> phone interview -> company interview -> final steps, so that indicates you have passed 1/3 or 1/4 of the hurdles. Due to my skills nearly every resume turned in becomes a phone interview, the phone interviews pretty much always result in an interview, but most interviews dead-end because they find "another candidate with more experience." If your situation is anywhere near similar to mine, then you would have 50% chance at best of getting the pentester job, possibly closer to 10%.
My advice would be to take the sysadmin position and leave the soonest you feel comfortable (from what I've heard you should be there a year, at least half a year, any less than that looks really bad). If you look for a job and have even a single year of job experience, that should work in your favor. You have already accepted it AND you do not have another offer.
Do not try to game the system, simply tell them you accepted another offer already but would like to consider them the next time you are looking for a job. If you say, "Well I was just hired by someone else so you better make me a really good offer to make me turn traitor to my current company and leave them," that reflects horribly on your character and they will run. If you tell your current boss, "Hey I know you hired me last Wednesday, but I'm thinking of leaving" that will also reflect horribly on your character and could damage future opportunities (You do know the next company you apply for could ask your current boss what he thinks of you right?).
My advice is just to forget about the other offer and pretend it never happened, stay with your job at least half a year, and at that point if you hate your job, maybe start looking around again and ask that company if you could interview with them.