I need some help with scheduling a procedures.

Here is the criteria:

Our business is service and installation.
We are a small company with 2 office workers and 3 field techs. 309 customers and 27 active customers out of the 309.(309 customers that have done business with us in the last two years and 27 on a regular basis) Most of our customers are commercial and a few are residential. Our commercial customer base is refrigeration and ice making.

Here is the problem:

Seemingly every time we are on an installation job we get called to a service call and have to leave the installation job.
This becomes frustrating for us and the customer.

I need some ideas for basic scheduling and some procedures for taking call and forwarding that to the field.

I am really frustrated and need some advice here.