There are two possible solutions:
1) open the phone (4 screws at the bottom plate) and unplug the rotary dial/key pad cable
2) the OPIS mobile phones come with some undocumented features such as: allow only short dials, block certain numbers, auto dial when the receiver handle is picked up.
As far as the latter is concerned, please have a look at the attached document. If you use MOD3 but do not actually assign a number to short dial *1 you should have the desired result.