Do you have an iPhone - or only an iPad?
If you have a DataSIM installed in your iPad, the associated Cellular Number is only used to identify your Cellular Account for billing purposes; it is not used to make or receive cellular calls or text messages. 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. An iPad is not, and can never be, a standalone substitute for an iPhone.
However, If paired 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 Appleâs continuity features. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity andare connected to the same WiFi network - but this is not the end of the story.
Now, if your both your Cellular Carrier and your calling plan support WiFi Calling, your iPad can make and receive calls independently of the iPhone - even when the iPhone is switched off, elsewhere, or using a different WiFi network. Not all carriers support full WiFi calling.
More information about Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad and Apple Watch â 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 â Apple Support (UK)
When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.
So, in conclusion, in conjunction with an iPhone, it is theoretically possible to configure WiFi Calling service for your iPad. After WiFi Calling is enabled and configured, your iPad can make and receive cellular calls whilst connected to a WiFi network (caveat: any associated firewall must not block WiFi Calling or VoIP services) - even in absence of the iPhone.
In addition to native calling features of iPad and iPhone, it is possible to use third-party App to make and receive voice calls using VoIP services. WhatsApp is one example - however, whilst WhatsApp have released an App for iPhone, there is not an equivalent App for iPad. Other services, such as Skype, are available.
As for Cellular messaging, if you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from your iPad you must explicitly enable Text Message Forwarding for your iPad on your iPhone...
On your iPhone:
Settings > Messages >
- Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices
- MMS Messaging - set to ON (if required)
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON