Hi,
Look at this only as recommendation because I did not connect STE phone but I think it should be standard.
You should add station xxx / where you fill
Type: WCBRI
Port: xxxx - any port of your TN556 location
BRI OPTIONS
XID?- n
FIXED TEI? - n
Country Protocol? - ETSI(or other country code, but must be programmed exact as in your phone menu)
Endpt Init? n (and set in your phone menu no SPID - Service Provider ID if possible)
After adding station check status station xxx - it should be in-service/idle.
And try to call...
Rgrds.
P.S.
May be you additionally will have to check the Companding Mode setting in the phone menu(A-Law or Mu-Law) and Definity switch country options. They must be the same to avoide terrible crackle during talk. Better change the phone Companding mode...