Contactreach has many ways for users to opt into a campaign though the majority use the "text keyword to number" opt-in method. That method requires you to use numbers with the country code of where you'd like to use them. Mainly, that is because users in a country text to send the campaign keyword to the campaign number. If you're demoing to Africa, for example, but use a US number, users may not be able to receive the Welcome Text Message as they can't send an SMS to the US number unless they spend for overseas SMS cost. However, other ways to opt-in are by letting users fill out the campaign form right away through the form's URL. They don't need to text the number but they'll receive the messages from the campaign through the assigned number. But if you're using an overseas number in that case, make sure it's global SMS-capable (other countries are enabled in the Messaging Geo-Permissions Page).
Although in some countries, SMS costs higher than using local phone numbers or toll-free numbers so it would always be best to use local numbers and toll-free numbers if possible. There are still many countries where Twilio SMS isn't supported yet and the only option would be to use overseas numbers. In that case, you can raise your package price considering the higher SMS cost.