Hi Richard,
To extend the ringing time before a call is diverted to Hullomail voicemail, please see this faq here:
https://support.youmail.com/hc/en-us/sections/1500000329642-HulloMail
If still having trouble, please send us an email at support@thumbtel.com with your account phone number and the team will be able to assist.
Kind regards,
Thumbtel Support
Richard Wiltshire