I think, that the question is, if the interrupt is called, when I am using long operations like e.g.
HSERIN 100, retMail2, [wait ($0D, $0A, $0D, $0A), STR COMMAND\96\$0D]

Becuase I have realized, that the HSERIN command is not probably terminated by the interrupt and it is waiting till the end.
So the only solution is to use e.g. longer TMR0 interrupt - like to switch the TMR0 to 16 bit, then the interrupt is called each half second and in that case it is always managed.

Are my assuptions correct?