Changin Job while on General Tier 2 visa

Hi There,
Currently I am working on General tier 2 visa, I have offered with another job and they sponsors licence as well. Since they are new to this they want to know the process? Here are some question
- do they have to provide unrestricted CoS?
- Do they go through RLMT for this job before they offer me?
-What if they are left with no unrestricted CoS
- Do I need to inform my current employer before I get CoS and Visa with new employer?
- how much time is takes to issue CoS