Slow ring time - external calls only

I have an issue where only calls coming from the PSTN have a slow to start an stop ring time.

When I call from the PSTN I hear ring back for two rings before my analog (or SIP) phones start to ring, then if I hang up the call before it is answered the analog (or SIP) phone continues to ring for 2 more rings?
When I call out this is not the case or with internal calls.

I am connected to the PSTN via a TDM400P.

Any thoughts?


Same with me (sangoma board). I think this is some kind of dahdi/hardware limitation.