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
Beware! If you have the carbon microphone on the right the pulse to DTMF circuit should work. The modern replacement condenser microphone on the left put more load on your telephone line, and the pulse to DTMF can't deliver a strong enough DTMF signal to overcome this.