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 5T review: Hands-on
Illustrator Andrés Lozano on his improv line work, brazen use of colours & hand sketching
iPhone X review
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