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Set up cellular service on iPad (Wi-Fi + Cellular models) - Apple Support
About eSIM on iPad - Apple Support
Be aware that the majority of mobile phone SIMs and eSIM plans are not correctly provisioned for use with tablet computers. If they work at all, they may not provide access to the full range of cellular services. This is not a limitation of the iPad, but of the Cellular network provider.
If using a physical SIM with iPad, you will require a DataSIM explicitly provisioned for iPad or mobile broadband (a.k.a. MiFi). eSIM plans for iPad follow similar criteria.
Be aware that an iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added an eSIM or physical DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the eSIM/SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages. An iPad is not, and can never be, a standalone substitute for an iPhone.
However, when coupled with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s Continuity features.
Continuity
Work across all of your devices seamlessly – Apple Support
SMS/MMS Messaging
Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support
WiFi Calling
Make a call with Wi-Fi Calling – Apple Support
Making and receiving calls on iPad
Make and receive phone calls on iPad - Apple Support
When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.