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I succeeded to make an AT connection to my mobile phone with a little circuit with no problem .. S35 phone for example has Speaker out and mic in pins but I don't know how to dial , carry voice on the pstn or detect ringing signal without damaging the phone ..
The comming noise is the confort noise sent from network to mobile when discontinued transmission is used or when no speech signal is going on between mobile and network. It could be important when duplexer is threshold activated ( let say hands free kind of implementation ) .But if hybrid transformer solution is applied and gain will be adjusted , it should not be big problem .
If you are trying dialin into to another volatile
in PSTN network , some tricks must be done about how to transfer destination number from mobile .
Of course caller Id will help you , but it is fixed .
DTMF tones can not be properly passed from network to mobile because speech optimized vocoder is used . May be prior to MS could be sent with destination number .
But no problems does exist with PSTN to mobile call where PSTN guarranties DTMF tones passing (if not DCME equipment - vocoder alike - is used between originating and destination entities ).
Have a close look to PSTN telephone's speech schcematics - it will give idea what to use in duplexer. The rerst is about handling AT commands from mobile .
You can start the dialing sequence, start/end the call, send SMS and much more via serial RS232 interface. Try to find the list of commands supporting by your specific mobile phone. You can than implement any PIC with serial interface in order to do this job.
For software side, it is the major the AT command which is easy available from the internet. There has a special trick to deal with ACI-phone (NOKIA) because of the two-line bi-directional communications. I also look for this kind of information.