When people open jam for 20BB, they usually have a capped range, because AA, KK and even QQ wants action and raise small. Most commenly this is going to be a middling pair like 77-JJ or AK. And for that reason this is a great spot to call with JJ. A lot of the time you will be a 4:1 favourite, and the rest of the time you will usually have 55%
equity against AK. Which is also fine, especially when its not for all your chips. Unless of course if SB exactly wakes up with a hand and decide to overcall. This will suck, but he has QQ+ 1,5% of the time, which is to rare to worry about it.