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 matter with my brother and we decided to try to make our own version for my GNT-A F68.
Update: We do not offer any support for this project anymore! Our best advice is to get a DTMF dialer app for your smartphone e.g. DTMF dial https://itunes.apple.com/us/app/dtmf-dial/id439867878?mt=8
Update: Good news head over our Telephone pulse to DTMF converter MKII for a working version of the circuit.
Click on the picture to enlarge it and read our comments and advice
This is a picture of the pcb after mounting all the components. I have chosen to use two screws as connectors for the cables from the rotary dial, as I will not harm the telephone if I decide to remove the pulse to DTMF converter again. Obs! The schematic is in the pcb zip file Update! Be aware there are two versions of the hex file: 1) puls2dtmf 0-9.hex for telephones where the dial starts with a "0" and ends with a "9" used in Swedish telephones 2) puls2dtmf 1-0.hex for telephones where the dial starts with a "1" and ends with a "0" used in Danish telephones