Raspberry 2 &3 are really powerfull and able to run heavy applications. Since more people are moving to raspberry and its chinese clones with 2-4gb ram, i think it will be a good idea to invest the building of issabel pbx for raspberry as well.
There alternates such as raspberry asterisk, but they lack the simplicity of issabels's pbx functionality. It lacks the look and feel of Issabel.
Please consider to create an image for RPI. It will be a really nice spread of Issabel to more people.