Talk to the Telco on this, but I doubt they have the ability to light a message waiting light, even on an off system analog phone equipped with a message waiting light. They probably only have the staggerred dial tone as a message waiting indicator.
As a further note, Telco(telephone service providor, local dial tone providor) voice mail incurrs a monthly service charge. Consider adding voice mail capabilities to your phone system, this cost is one time charge for installation and equipment, and also has exponentialy more features available for your usage. Including mailboxes for as many users as you want in many options, notification calls of msgs. waiting, and a myriad of other features. It could very well pay for itself in the cost savings over time, and would be unusual if it does not.