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I want to dial a phone number on a landline with DTMF tones generated by my phone. I tried soldering a 3.5mm audio jack to a phone cable and using my regular phone app, but nothing happened. In this video, this is done by simply playing the tones in to the speaker, but that wouldn't work on my phones either.

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  • \$\begingroup\$ does your landline support DTMF tones (it probably does, but since chances are it's a NGN software-defined landline... Maybe it's restricted to impulse dial) \$\endgroup\$ Jun 21 '20 at 20:04
  • \$\begingroup\$ it's not about the phone, it's about the exchange \$\endgroup\$ Jun 21 '20 at 20:09
  • \$\begingroup\$ also note that your DTMF tones need to be pretty loud when you play them into the mic of your phone. \$\endgroup\$ Jun 21 '20 at 20:10
  • \$\begingroup\$ ah wait, if its a modern wireless handset, it's very likely not your handset that does the dialing, but the base station thing. Hence, that probably won't work. Directly going to phone cable won't work either, usually. \$\endgroup\$ Jun 21 '20 at 20:11
  • \$\begingroup\$ I'm going directly to the jack in the wall. \$\endgroup\$ Jun 21 '20 at 20:14
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I tried soldering a 3.5mm audio jack to a phone cable and using my regular phone app, but nothing happened.

No, that wouldn't work because you need to take a DC current from the line to signal to the telephone exchange equipment (SLIC or subscriber line interface card) that you are "off-hook".

Putting a resistor across the line to draw circa 25 mA would correctly signal to the SLIC that you are off-hook. Probably about 1 kohm would do the job.

But you are in danger of busting your phone because the open circuit line DC value is 48 volts and can be superimposed with up to 90 volts RMS ringing alert current. You might be seeing a peak of 180 volts. Your phone may not like that.

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I tried soldering a 3.5mm audio jack to a phone cable and using my regular phone app, but nothing happened.

You can consider yourself lucky. The PSTN system works on 50 V DC and the ringing voltage can be up to 100 V AC (20 Hz, I think). Both are enough to destroy your phone if you connect your phone to a telephone socket (which is what you seem to be describing).

In this video, this is done by simply playing the tones in to the speaker, but that wouldn't work on my phones either.

You haven't described your landline system. If you hear DTMF when you press the keypad then it is most likely to use them for dialing and playing DTMF on your mobile phone's speaker while held up to the landline microphone (not the speaker which is the earpiece) should work.

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  • \$\begingroup\$ I hear tone pairs when I press keys on the landline phone. \$\endgroup\$ Jun 21 '20 at 20:17

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