There is no easy way to capture a mobile subscriber from CloudPages. The only current two options are:
- Display the shortcode and JOIN keyword and have the end user text the keyword.
- Use MobileConnect API (specifically QueueMO with is on the deprecated v1)
There should be an integrated equivalent of Smart Capture available in CloudPages but for SMS mobile phone numbers. This should include the list of Keywords created in MobileConnect and their corresponding shortcode.