I take it you want the CO to ring the SLT extn and ring to the CO keys on the DKT handsets.

This can be done by setting the CO keys to ring on the DKT handsets and in P310 set the PSTN trunk to dialing digits and enter the destination of the SLT.

The call will be presented to the SLT and ring on the line keys on the DKT handsets at the same time.


Regards
Carl