Just stay focused and go broken record. I would be tempted to say the phone is not working or there is no mobile reception at the foster carers house!
Introductions are full on, they are exhausting. When not 'on' so to speak you are back home resting, not running a shop!
Please tell your dh to stand up to her, and to make it clear. If 85 year old grandma is wheeled in to do the job, so to speak, let them get on with it.
I could not quite understand all the bits about your existing child but he will need both time to bond with new baby/child alone with mum, time alone with dad. If in the middle of that there is time for granny, so be it, if this is nice for him or helpful for you. The needs of your two kids come first and not the older generation.
But be kind too, they are family and they may be of 'use' (in the nicest possible way!). No need for confrontation, or angry words, just be clear and un-contactable; be upfront, 'We won't be contactable all week, we will call you with an update soon.'
Good luck. Enjoy. 