Kazz, you can not expand a TD308.
Lets start by finding out exactly what you have.
There are a number of things that can keep a MWL from lighting.
usually it's in the mailbox settings where "all calls transfer to mailbox" is set to yes, this has to be set to no. It may as simple as not having the MWL activated in the individual mailbox.
Can you access line 5 at all through the system?
This would be the 1st line on the KXT180 expansion card. Do you have outgoing privileges assigned for line 5 on that port?
It sounds as though it may be in your best interest to bring in a certified Tech. at this point and just pay for a simple 1 hour service call but none the less check these out and get back to us.
MrG
