Try going old school, ask them to send a post card or letter with an activation code you provide them upon receipt. Banks do this in lieu of sending verification codes to WiFi phone numbers. WiFi phones, i use a textnow number for survey registration, are not considered a "real" number and sending verification codes maybe block. I had to do this with a bank I used to agglomerate PayPal rebates from Amazon reviews. If support balks at this, ask to be escalated to the project manager in charge of support. If they still balk, then skillful research of business directories pertaining to the business would be next. Directories like Linkedin, crunchbase, corporate registration, County DBA records, BBB, professional registrations in Marketing associations, think ESOMAR, and rocketreach, to name some. This may seem like a lot of work for something that might not be worth much. Think of it as self-guided training for when some panel decides to really screw you, one of them will. If you latch on to them like a rottweiler with lockjaw, until you get your due. I've done this a couple of times and it takes some familiarization with the limitations of the freebies.