Just three small points:
1. There is no need to enter any real number after +372 (but do not change it) I believe normal OneSim number should have 8 digits after 372, so this is the format I recommend. And as you do not to enter any real phone number and even email is optional - I can not see how it can be a scam. All they get from you is your name and FF number.
2. I have only started this thread after receiving miles/points for different accounts on M&M,FB and Priority Club. All posted within 1-2 days from each other. Used different phone "numbers" for sign up.
3. If you want to get real miles for roaming - go the proper travellingconnect site.