Thank you all, but then what is the correct answer? Is it a current variation(if so, how do I change this current to voltage) or voltage rms from 0.2 to 0.7 rms? Because I want to do ADC for this audio voltage. Could you please help me step by step? I'm new to this kind of stuff.
Tom
The voltage available at terminals A and B will depend on the number of turns the nnnn winding has, and to the audio current in the telco line. The audio voltage in the telco line is in general words, 0.2 to 0.7 VRMS
Choose a transformer that can handle at least 50mA DC in the ~8Ω uuuu winding, the secondary winding nnnn can be any low DC current rating and try to find a low impedance or number of turns.
Edited/added: ---->
Audio Output Transformer - RadioShack.com
The ringing voltage can put out over 100 Volts AC in the terminals A and B; if the transformer is inserted between the telco and the ringer.
A couple of inverse parallelled diodes to those terminals can limit the spikes, ringing or audio amplitudes to a safer 0.7V PP
If transformer insertion point is protected by a hook switch, the terminals A and B will not be exposed to ringing voltages and can feed your ADC safely and directly.
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I do not understand the
... "You need to add a resistive load between A and B otherwise the transformer will act like a choke and suppress the phone signal."...
Resistor o no resistor, the transformer will always present inductance to the telco current; and in order to 'choke' the audio, it would have to be a huuuuuge impedance of thousands of ohms; which is not the case here.
Miguel