I am having problems establishing a point to point data call with an Iridium terminal.

Each Iridium sim card comes with two MSISDNs.  MSISDN-A is the standard phone number supplied with a sim card for voice and sms.  MSISDN-C is the data number.

If the Iridium terminal is configured as a modem to receive and send data, then you will need to dial into the terminal's MSISDN-C number. (This is not an Iridium internet access number).

The sim card/service provider can give you the MSISDN-C.

Also, make sure that the terminal is capable of auto-answering incoming calls if unattended and set to auto baud. Some remote terminals do not respond to auto-baud quite well and therefore it is best to try different baud rates until you can establish a PPP connection.