You can edit a contact's phone number in two ways. The first is through Contact Details.
You can also edit the phone number through your Address Book by selecting Address Book > View> making the updates you need > Save.
If you had already started a conversation with a contact and then edit their phone number, any messages to and from the updated phone number will appear in the original Conversation History in your Textline Dashboard. The contact will receive all messages moving forward to their updated phone number.
Please note that the system will still not allow for duplicate phone number entries in the Address Book. If you attempt to change a contact's number to one that already exists in the system, you'll receive an error message.