You have one trunk line at the moment. This means that only one external connection can be made each time. When the trunk is busy, other calls cannot be done (in your case I assume that they are calls to / from POTS).
Other functions of the PBX will work correctly, considering that you are not trying to use the trunk or the busy extension.
Asterisk can be really flexible about handling busy and other situations (i.e. forwarding calls to free extensions, voice mail, ivr and so on).
Regarding the X100P board, I tested in the past with a generic intel modem (there are some modems that have the exact chipset of the X100P card). This worked at that time, and maybe you can get it to work now too. It's just a matter of finding the right modem. Unfortunately I don't remember what was the modem chipset.
