See how it works (sample input 2):
These are memory cells:
0:CALL 3
1:CALL 3
2:CALL 3
3:BELL&RET
Before operatons stack is empty
First the first command is executed. Stack becomes (1),IP-3
BELL&RET is executed. Then IP pop-ups to 1 (from stack).
The second command is executed. Stack becomes (2) and so on.
The result is 4 executed commands 'BELL&RET'

i see now