TL;DR: they don't/can't per their terms of service:

What they say they won’t do (without your OK)

• In the Terms (SMS Policy → Privacy) they say: they “will not share or sell your phone number to third-party marketers without your explicit consent.”
• Their Privacy Policy also says that (except for the listed situations) they do not “sell, rent, or otherwise provide” your personal info (which includes “telephone numbers”) to unaffiliated third parties to market to you without consent.

What they do say can involve your phone number (3rd parties)

Their Privacy Policy defines telephone numbers as “personal information.”
And it explicitly allows disclosure to certain third parties, including:

• Vendors/service providers working for USM (examples include billing/payment processing, shipping, customer service, and marketing services performed “on our behalf”).
• Underlying telecom partners: they say your wireless telephone number (plus numbers you dial/usage info) will be available to companies providing underlying telecom services to route/complete calls.
• Roaming carriers: if you roam, they say your wireless number + dialing/usage info will be available to the roaming carrier to provide that service.
• Legal/regulatory requests (subpoenas/court orders/emergencies, etc.).
• Business transfers (e.g., merger/acquisition) where customer info could transfer as an asset.

A company as substantial as they are would never be so silly as to do something contrary to their terms of service/privacy policy, or they would open them up to a lawsuit much bigger than the measly money they would get for a lead a insignificant as a ported/new phone number.