How do you accept or decline work orders from a functional manager that pull you away from your critical PM activities?

First, it is assumed that there is a coordination and cooperation between project manager and functional manager through clear vision and common understanding.

Second, it is assumed that there are governance rules to follow and obey.

Otherwise, we plan to fail

If pull you away from the critical activities considered as a favour, You should accept the job order.   

If it is a kind of insultation then decline the job order is recommended.

I love the answer given by Mr.: Mohammed saei &  i do agree totaly with his mind

we must have the clear vision in the all work & the co operation too