Good morning Richard,
We do not have an option to customize ringtones for individual contacts, however you can make your ring tone different for text than for calls.
1. Open the TextNow application settings
2. Select Notifications > Notification Sound > Select the notification sound you want for texts
3. Go back to TextNow Settings > Calling > Ringtone > Select the ringtone you want for calls
Once you have made your selections you should be all set!