Is it possible to use my regular (old) telephone as a speak/hear-device (sort of headset) with softphone/voip-software through my old dial-up modem? Something like the modem converts the analog audio signal from the telephone to wave?
Very few modems are able to work this way even if you have the software to do this. Best bet is a special usb phone or a phone adapter that connects your phone to the internet without a computer. Digium modem cards and asterisk is the reason the answer is not "no". This is way more than most people want to do and adapters/usb phones can be found for less than $100 usd.
If it's a regular telephone then you need to use an ATA device. This provides a 'dial tone' to a regular telephone. Also know as an FXS port. They're generally around $90. Others can be had that will connect an old phone to the USB port of a PC. These are around $50 but require the PC be running to use the phone, whereas an ATA is stand-alone.
A 'old dial up modem' has little use for VoIP activities.
Maybe you could adapt an old handset from an analog phone and plug it into the microphone and speaker jacks on a full-duplex sound card. That, with the softphone software might work. I haven't seen any information that indicates that VOIP is doable with a standard dial-up connection though. That said, I would just get a cheap headset instead of trying to reinvent the wheel, especially since the weak link will be the dial-up connection.
You could always download one of the free softphones and give it a try with your dial-up line.
I will prefer use my old phone instead of a headset - the phone is made for the purpose. I will try making it go into the sound jacks
The headset about 600 ohms should work ok on the soundcard but the mic if carbon element may need some matching.
This has some good ideas:
Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.