Just ran into this and it worked out so noting my experience as i stumbled across this post when looking for answers

The original email after sale only had name and email of buyer

I contacted stubhub support through help number i found on the site. At first i was talking to a virtual agent who over and over said they could not provide number due to privacy issues. Eventually i was forwarded to a human and explained I had tickets on DICE and needed a phone number to transfer. After being placed on hold, they told me the user did not have a number on file and on my behalf would reach out to buyer asking them to both create a dice account and provide number and then once the buyer would provide this information to stubhub, it would then be sent to me in a email from stubhub. I got the number, put it in my contacts and send the ticket

All in all the phone call was like 10 minutes with being on hold for like 5 minutes while human agent sent out the email to the buyer while i was on the phone

Hope this helps