SMS: It depends on your country.

• For Germany, UK, and The Netherlands, you’ll have a shared number available with which you can create and publish an SMS flow.
• For all other countries, a UK Number will be assigned automatically unless not supported by local restrictions—as some countries overwrite the sender ID—in which case you will indeed need to buy a Dedicated Number. 

Voice: Yes, you need to buy a Number to be able to publish your flow.

Webhook: No, you do not need to buy a number to be able to publish a Webhook flow. If there are SMS and Voice Steps inside your Webhook flow, the same requirements as above apply.