I have a Polycom phone going thru an SLT adaptor. I cannot create a conference call with 2 external parties. Example:

I make a call from the Polycom phone, press flash, get system tone, dial 9, and I get a slow busy.
I can however, make conference calls on the digital stations.

Also, I can create a conference with 2 internal and 1 outside party.

Try creating a conf using your buttset on an analog port. I normally don't have any problems w/ this.

I saw in the NEC docs that you cannot create
a 2 co line conference with a Single line..

for all who cares (and yes i see the question has been answered)

per the features & specs manual

to do a conference call on a analog port
1. Press the hookswitch to place the first call on hold.
2. Dial an internal station and announce conference.
3. Press the hookswitch again. Talk with both parties.

and the restriction:
A Single Line Telephone cannot be used to originate a 2-party CO
conference.
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this limitation was in the 48,192 and now it's still in the ipk