TeleSign offers multiple products that can help with verifying a user. If you want to verify that a user's phone number is legitimate, you can use the SMS API or SMS Verify products to send an SMS containing a verification code to the phone number they provide.
You can use the Voice or Voice Verify product to send a voice message containing a verification code to the phone number they provide.
You can also use Auto Verify to provide a frictionless end user experience. The end user provides their phone number, and then Auto Verify checks that the phone number is good by placing a call or sending a text to the provided number and automatically handles verification.
Besides these products, you can use PhoneID to collect information about a phone number provided to you. Additionally you can use Score to assess the risk of a given phone number.