My other phone is forwarded to my mobile phone and callers must enter their 10 digit number

Your callers will be requested to enter their 10 digit phone number any time Caller ID information is not associated with a number in your YouMail account. In this case YouMail is receiving the Caller ID from your home/office phone. Since this number is not associated with your account we cannot identify the applicable YouMail voicemail box.
To solve this problem, simply add another phone to your YouMail account.