If it's not optional I usually fill that part in with zeros, mostly that's accepted. If not I invent one.
For those that really do not have a mobile number I guess they have to concoct one as well. What fun :)
OnePlus 5 review
Alice Saey's mesmerising animation for Dutch singer Mark Lotterman
iPad Pro 10.5in (2017) review
Comment booster votre iPhone ?