Can you limit inbound calls from the PBX itself? It wouldn't matter who provides the PRI in that case.
To do this on the TA908 I see three possible solutions, one on the SIP side and two on the PRI.
On the SIP side, in your voice trunk type sip, you could try 'max-number-calls'. Create one trunk for inbound SIP and another for outbound SIP, apply the max to the inbound trunk only. Set dial plan to/from the PRI accordingly. This method allows either side of the PRI to seize any open channel.
On the PRI side, you could use separate isdn-group for inbound and outbound. Use 'max-channels 10' on one for inbound, and maybe 'min-channels 13' on the other for good measure. Then create two voice trunk type isdn, one for inbound and one for outbound, and 'connect isdn-group' to the appropriate group. Set the dial plan to/from the SIP trunk accordingly. This method also allows either side of the PRI to seize any open channel.
The other PRI method starts at the underlying T1 interface: create two tdm-group using different timeslots, e.g 1-11 and 12-24. Then create two separate PRI interfaces using "connect t1 ..." to each tdm-group, and two isdn-group and two voice trunk type isdn per the above. Set the dial plan to/from the SIP trunk accordingly. This method basically gives you two fractional T1 at the cost an additional D-channel, and the two trunk groups also needs to be configured on the PBX so it will seize the correct channels if it will even support this. I don't know why anyone would want to do it this way.