Hey @tgriffiths thanks for your post! I can see that you have a support ticket open with Anna from our technical team, I will go ahead and share her response here as well just in case anyone else is looking for help with a similar request!

If you're looking to send lead data to a third-party server after a form is submitted, you can use webhooks: https://documentation.unbounce.com/hc/en-us/articles/203510044-Using-a-Webhook.
 
We also have the "Post form data to URL" option https://documentation.unbounce.com/hc/en-us/articles/203708344-Getting-Started-with-Integrations#content3. Out of these two, I recommend webhooks because the request and response for each lead sent via a webhook are recorded and available to view in the leads tables. This makes it easier to troubleshoot compared to "Post form data to a URL," where there are no logs.
 
It's not possible to add server-side scripts to Unbounce. We allow only frontend Javascript and HTML to be added to pages:
 
https://documentation.unbounce.com/hc/en-us/articles/203879070-Adding-Custom-JavaScript-and-CSS-in-the-Classic-Builder 
https://documentation.unbounce.com/hc/en-us/articles/360035250491-Adding-Your-Custom-Scripts-Using-Script-Manager 
https://documentation.unbounce.com/hc/en-us/articles/360034877112-Embedding-Custom-HTML-or-Widgets-into-Classic-Builder 
 
It is possible to send HTTP requests from Unbounce pages with scripts you add though.

Yes, it's possible to build a landing page in Unbounce with a phone number form and integrate it with Segment CDP. You can use the platform's custom JavaScript functionality to send form submissions to Segment via an HTTP API. After capturing the phone number, you can use Segment's tracking to send data server-side. Be sure to configure the form's submission event to trigger the API call.