Is it because:

1. They are waiting for THE call from MRS to resume trading and the

call display shows it's NOT MRS or their lawyers or the tax dept.

2. They want to appear busy, so do not answer the phone (but see #1)

3. They are really out of the office/out for lunch/out for coffee

4.  Whoever answers the phone is sick/had a tooth extracted and cannot talk 

5.  ??