God, a holiday with your in-laws, sounds like fun (not).
Letting your phone go through to answer machine is practically obligatory when you have a small baby, in fact they should sell special phones that don't ring
(I have a brilliant setting on mine called 'do not disturb' which meant I could select which callers would be allowed to get through (i.e. my mum) and which callers would go straight through to answer machine (i.e. my MIL).
With the taking him off you when he cries thing, I think I would just firmly say "thanks but it's better if I just take him out of the room for some quiet time to calm down" and then do just that. (Another possibility is: "oh dear, the last time he did this he started throwing up all over the place, you'd better hand him back" - ha ha!)
I think it is totally unacceptable to take a crying baby off the mum unless (a) she is hysterical or (b) she asks you to. Mums needs to be with babies as much as babies need to be with mums when they're upset.
Agree with the others, v important that you and dh are united on this, and that dh take the lead really in keeping her under control.
Please don't let this keep you awake at nights, definitely postpone seeing her whilst your dh is away if gives you peace of mind. Is it worth talking to your SIL about how she's learnt to cope with your MIL?
I tend to keep mine occupied with jobs, so she doesn't get any funny ideas, so suggest she take ds out for a walk in the avo to give you a break, or something similar, maybe? Or ask for her advice even when you don't really want it - i.e. with the crying, maybe say "what did you find worked best with dh at this age?" (Generally she will not be able to remember anything specific, which will keep her on the hop anyway!)
Remember, the most important people are you, ds and dh. Don't do anything that is going to increase your stress load, you have enough to worry about. (And frankly that includes this holiday, although at least you'll have your mum to run interference for you).