I have an MBX system up and running , the customer wants to add single line phones, the SLT ports on MPB100 give me a fast busy when I plug in my analog set any suggestions or words of wisdom from any with the same issues?

Are they Hot Lined to some invalid destination such as an empty CO Group?

If they're Warm Line you should at least get intercom; set it to that if you don't mind putting a 9 calling out.