"An alternative to do the testing (I think) is to make a connection between two telephone units ... Can the modem do the task?"

No.

Modems (or telephones, for that matter) can't just be connected "back-to-back" - you need a telephone exchange (switch) between them.

You need an exchange simulator; eg,
http://www.epl.co.uk/
http://www.chesilvale.textron.com/