Why does comcast phone go straight to voicemail?

When your iPhone is locked (the screen is off), Do Not Disturb silences all incoming calls, text message notifications, and alerts on your iPhone. Unlike silent mode, Do Not Disturb sends incoming calls straight to voicemail.
https://www.payetteforward.com/why-does-my-iphone-go-straight-to-voicemail-heres-the-fix/

Remove the battery and allow the set to phone to reset. Now check for the ringer setting at teh back of the base phone to be set to the maximum rings, there can be three settings.
Some times this is also set on the menu.
Connect and test the timing to confirm

check your incoming calls settings and see that you did not choose it to divert to voice mail number

Please check if any call divert or incoming call barring (found at settings menu) are actives .

Per the Sprint support site at http://search.sprint.com/inquiraapp/ui.jsp?ui_mode=answer&prior_transaction_id=2&iq_action=5&answer_id=33554455&highlight_info=6293864,0,6&turl=http%3A%2F%2Fsupport.sprint.com%2Fsupport%2Farticle%2FSet_the_number_of_rings_for_voicemail%2Fcase-ev207663-20090626-084943#__highlight&id16=voice+mail :
"A call will roll to your voicemail if unanswered after 23 seconds.
This setting can't be changed. However, on your Sprint phone, you can change your ring tone to one that reduces or increases the number of rings you hear before an incoming call rolls to voicemail.
For details about setting up and changing your ring tones, refer to the user guide for your phone."
If this is not the problem with your phone I suggest you call their support line:

Call *2 from your cellular phone or 888-211-4727 (Sprint phones) or 800-639-6111 (Nextel phones)
They may ask you to perform troubleshooting tasks, so it would be best to call from a phone other than the one you need help with.

check in settings/phone to see if you have maybe turned on call forwarding and it is being forwarded automatic to your voicemail box number

You please change the setting from answer voicemail to ring mode if you have any dificaltey refer your phone user manual book and reset the option

do a master reset and clear of the phone

i would got into your voicemail settings and check to see if you have it set to ring once then straight to vmail.. i'm not sure how your phone is set up completely but i would assume you can adjust the amount of rings before vmail picks up.. is this phone new to you?
hope this helps..
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get your mailbox number then type in **61*44then mailbox mumber*11*then a two digit number between 00 and 30 (seconds delay).