home phone drops call after exactly 30 minutes

... How and where do I split the coax line and directly connect the home phone ...

The coax line does not connect directly to your telephones. Coax connects to a modem or gateway that includes telephone jacks, and your telephone lines connect to those jacks. The voice modem or gateway converts back and forth between the analog POTS signals on the telephone line and the digital VOIP signals on the coax line. You can't connect phones directly to the coax.

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Hello, @user_7vao6x. Thank you for reaching out over Xfinity Forums for support, you have contacted the right place for assistance in regard to your Xfinity Voice call concern. Hopefully the information that our great community expert BruceW answered your question with connecting directly to a coax cable. 

Investigating I did find an older post where users were experiencing the same issues with calls dropping, a resolution that was posted by a user was to get a splitter for the dual connection: https://forums.xfinity.com/conversations/home-phone-service-equipment/call-drops-every-30-minutes-using-xfinity-voice/602db105c5375f08cd4213e2.