It depense on what type of Ericsson you have. Is it a MD110?
Try te get hold of a IP card will make your live simple.
You could also do this with the S0 bus. enable 2 of them on each site and connect them with 2 BRI Vwic card. This way you can also use compression.
If you use FXO cards than you will have to use for every line 1 FXO card. That meens that you need a big router to make it happen.