I would think the problem would be in the PBX IP card. The AS5300 would process any calls
received over IP and sent out through the PSTN
via the T1 or E1 PRI installed.
There are 24/30 channle for T1/E1 respectively so the AS5300
would be able to process simultanously 24/30 calls depending on which
signalling type either T1/E1.
I would check the IP cards to see if its well configured to forward more than one call.
I'm not familiar with the card mentioned but can assure you the AS5300 would forward any calls received
as long as there are free DS0's.
debug voip ccapi inout to see if other calls actually arrives
at the AS5300.