Because YouMail utilizes your carrier's Conditional Call Forwarding feature to receive your calls, we have no control over the call until the message goes to voicemail. This means that you'll need to contact your carrier in order to change the number of rings before the call ends.
Some carrier's have disabled the ability to change the number of rings though it never hurts to ask. Contact their Customer Support or Technical Support Teams to find out their policy on the matter.