Be aware that you get absolutely no guarantee that the public internet will deliver your delay and packet loss sensitive data to the far side. Unless the service provider can give some kind of service level agreement, any QOS set up on the routers will not help if the packets get lost in the middle.
The public internet is great for data but in many places not good for voice and the ISP's won't be interested in helping out.
If you can live with this risk, then the best solution would be a PRI card in the PABX and matching PRI interfaces on the routers. You can easily configure a fractional E1/T1 for 10 timeslots. Trying to put in 10 analogue interfaces would take up too much space in the chassis (for reliable analogue trunking between PABX's , use E&M interfaces , FXS/FXO lack adequate answer and disconnect supervision)
