MATZ
I once called up Brize requesting a transit and got the odd reply of a clearance (with altitude) and 'remain clear of CAS' Couldn't get a call back to clarify and climbed over the top.
Rang the controller next day and he commended what I had done but to keep trying to call them as he could have advised another a/c to go over the top on a reciprocal heading.
If you have box 2 you can always tell atc you are giving the matz a quick call and will be straight back.
MM