Normally to accept a call you would provide a 600R load, made up from the transformer primary winding resistance and impedance, but the transformer must be able to handle the 50mA or so of DC current that will flow through it. Commonly these days you use a transistor constant current circuit ( and a bridge rectifier so it only has to handle DC, and does not care if the line polarity is reversed for some reason) to pass around 40mA of current, which does the off hook emulation, and the constant current circuit gives a high AC impedance so the audio signals are not going to be attenuated by it, and the capacitors on the transformer primary still provide DC isolation. The constant current circuit however has to handle up to 200VDC on it, as it has to provide the current sink even during a ring cycle, as the system provides from 60-160VAC 25Hz ( for the UK) to ring the bells, riding on top of the -48VDC on hook line voltage.