No, you can't.
Yes. As long as you are roaming with a Digi Prepaid™ roaming partner and have access to the *128# Quick Menu, you can perform a Talktime Request.
No. You can only send Talktime Requests to Digi subscribers.
You can request for a Talktime Transfer™ of RM3, RM5, RM15 and RM25. You can also request for the person you are trying to reach to call you back.
You can make up to three (3) Talktime Requests in a day.