Though I’ve actually now had the assessment and award for 10 years I was in a similar situation in July of needing confirmation from them to show the minimum 12 months which they were happy to provide and I received their letter the following week.
Regarding calling them I’ve found repeat calling as they are about to open, or phone anytime and get on and do something else while you’re waiting for them to pick up.
Remember that it makes a nice change for them to receive an appreciative call that respects that any delay/issue with the application is unlikely to be the fault of the person you are actually speaking with. They are human.