I am in the process of proposing a UC520 installation, but I may need several FXS ports. I will need about four FXS ports to support an existing external paging system, as well as a few more to support some analog fax machines. I believe the 48 user UC520 has 12 FXO and 4 FXS. Are any of the FXO ports in this model removable, so that we can replace them with FXS? Or would it be better to use ATAs to connect to the analog devices?