During a visit to the local recycling station I stumbled over an old GNT-A F68 telephone, and it was a bargain I could not say no to. When I connected the telephone to my Sipura SPA 2102 IP telephone adapted, I realized that I could not make any calls from the telephone as it dials through pulse and my adapter only accepts DTMF.
Surfing the web I found a design similar to the one below but for a Dutch T65 telephone and this model is somewhat different than my GNT-A F68. I then discussed the mater with my brother and we decided to try to make our own version for my GNT-A F68.
Warning! If you have a modern condenser microphone in your rotary telephone, this circuit will most likely not work, take a look at the pictures for more info on this.
Update: We do not offer any support on this project any more! Our best advice is to get a DTMF dialer app for your smart phone e.g. DTMF dial https://itunes.apple.com/us/app/dtmf-dial/id439867878?mt=8
Click on the picture to enlarge it and read our comments and advice
The pulse to DTMF converter connected to the telephone. I decided to place the pcb in a shrinkable tubing and holding this in place inside the telephone with cable ties (I have not taken any picture of this). The wiring is shown in the schematic, but be very careful as you risk to destroy your IP telephone adapter, and NEVER connect a telephone with my pulse to DTMF converter to the PSTN telephone system, it is illegal.