Android - Is there a way to completely disable the dialer including emergency calls to make an old phone a "tablet"?

If you root the phone, go into Manage Apps and you can disable the dialer, it will remove it from the home page. Contacts can also be disabled. There are also numerous ways to put the phone in Airplane mode so it's persistent after restart.

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NOTE: disabling dialer does not disable the emergency dialer, but airplane mode does.

On rooted devices, you can disable the following packages to disable all telephony features:

• Phone
• Dialer
• TelephonyProvider
• Contacts
• Contacts Storage
• ContactsProvider

Depending on the device manufacturer and OS version, some of these apps might not exist, or be named differently.

Does it have a removable SIM card? If so, remove the SIM and it won't have any network connectivity except for Wi-Fi. That's what I've done to turn my old iPhones into iPods for my kids.