If you look at the feature matrix it will give you more detailed specs:
The unit is spec'd at 100 SBC calls through the unit. I would not combine 100 simultaneous SBC calls plus 20 PRI calls through the box. I would not sell a unit where it is initially maxed out.
The box you need to use in that solution is the NetVanta 6250. It can do up to 200 calls with up to 4 PRIs.
The other option is to put the NetVanta 3140 in front (300 SBC calls) and then daisy chain via eth to the 908e for the PRI connection.
Hope that helps.