Every week for the last five or six years, Scott Isdaner got another call: Are you ready to sell? As managing partner of one of the last remaining larger independent public accounting firms in the Philadelphia region, Isdaner, 67, had watched many of his competitors — “our brethren” — get acquired by national companies, in many cases because they needed to do so to survive. Last week, Isdaner finally joined their ranks — but from a position of strength, he says.