Haucke & MGO,
That part is simple to do.
Go to System > Devices and Feature Codes > Numbering Plan > Dial Rules
Find the first unused Dialing Rule, by default 8.
8. Add *15
9. Add 999
Then go to your Facility Groups > P15xx > Dial Rules
Right-Click to Add a Dial Rule then add either 8 or 9 to the Dial Rules where you would need them. If it is required to be in front of the dialed number then make sure you drag and drop it to the top.
Example: Local Facility Group P1500
Dial Rules
8 Add *15
3 Echo 3-digits After Toll Field
4 Echo Local Address
This will dial *15 and then the 3-digit office code followed by the 7-digit phone number.
That should get you what you are looking for, but you will need to do this for each type of call, read facility group, that needs to have the *15 or 999 added to it.
Thanks,
TE