What is the effect of the final telephone call upon the characters in 'An Inspector Calls'?
The final telephone call is necessary to reinforce the inspector's message with the remaining party members who feel they have escaped the punishment for their sins.
Eric and Sheila have both learnt their lesson with regard to how they treat others by the time the 'first' inspector leaves. Sheila says
SHEILA: ...You turned the girl out of one job, and I had her turned out of another. Gerald kept her - at a time when he was supposed to be too busy to see me. Eric - well, we know what Eric did. And mother hardened her heart and gave her the final push that finished her. That's what's important - and not whether a man is a police inspector or not.
Similarly, Eric sees that the inspector's identity is not the issue-
ERIC: Whoever that chap was, the fact remains that I did what I did. And mother did what she did. And the rest of you did what you did to her.
It is necessary to have the phone call at the end of the play as the impact of the description of the death of Eva Smith was not enough to show Mr and Mrs Birling the error of their ways. The inspector had highlighted that there were, and are, 'millions' of Eva Smith's. The implication is that the Birlings' treatment of others will return to haunt them in one guise or another.