None. Install VOIP app and use buds. iPad Mini 6 or 7 becomes a great phone- no additional spending required.
Make it a cellular Mini and it is a 5G "phone" too that costs significantly less in both hardware and for cellular service.
I've been doing this for about 10 years now, since Mini 2. No problems at all. Works great for my purposes.
Doubters? Install VOIP app on any existing iPad (or even iPod touch if you still have one). They will usually give you a phone number for free. Then call your phone and/or use your phone to call your iPad (yes it will ring, just like a phone). Text your phone number and reply back to your iPad. It "just works" fine because telephony is only an app... just like other apps can make an iDevice become a flashlight, camcorder, tape measure, camera, voice recorder, video editor, iPod, etc.
BONUS: iPads tend to have long use-life legs, unlike iPhone which seems to suffer the "long in tooth" effect in only about 3 or so years. Wonder why? Since Apple somewhat ignores iPad mini, the 7 is probably the "latest" one for the next 2-3 years and thus, they won't be slowing it down while it is the one currently for sale. I rode that 2 for EIGHT years- try that with an iPhone. I expect to ride the 6 until about 2029-2030 at which time, I'll move on to the cellular 9 or 10.