SMS marketing is a great way to send time sensitive messages to your contacts that average extremely high engagement rates. There are a number of different phone number capture apps in Hive you can leverage to collect phone numbers.
When creating your signup form, check the Phone Number box under "Form Fields" to collect phone numbers when someone signs up for your mailing list.
Note: A subscriber will only be opted-in to SMS if they check the "I consent to receive automated marketing by text" box when providing their phone number.
Once installed, phone numbers from your signups will automatically be synced to Hive and will be available on the user profile and in the contact list filters.
You can find the developer docs here: Hive SDK Docs