This is not a forum for Elastix. The Elastix support forums are at elastix.org/index.php/en/component/kunena/
This is a GSM terminal, not a cellular phone. That page indicates it has been discontinued and does not clearly indicate how it interfaces to the PABX, but it is likely that you treat it as a landline connection to the PSTN, i.e. as an FXO line.
If you don’t know anything about the software you are using, you should read the documentation, take a course, or employ a consultant. Peer support forums are not appropriate places to ask for step by step instructions.