Yes, you can, although functionality may be limited.
iMessage
Apple’s Messages app can send messages as iMessage or SMS/MMS. GigSky's service is data-only, so you will only be able to send messages as iMessage when using a GigSky data plan on your iOS device. When you create a message, your phone will show you a list of addresses from your contacts. With GigSky's data-only service, you will only be able to send to those contacts with an email address shown in blue. You will not be able to send messages to the contacts that only have a phone number in blue, or that are greyed out.
This will work regardless of the fact that the Messages settings screen displays a "waiting for activation" message. The Apple system is trying to send you an SMS to activate that part of the iMessage functionality, which will only be received if you are using a SIM card with voice and SMS functionality.
In summary, you can use the Messages app with a data-only service, but only to send messages to other iMessage users with an email address, and not iMessage users with only a phone number. The same goes in reverse - you'll only be able to receive messages from iMessage users sending messages to your Apple ID email address.
For example, if you’re already using iMessage and swap out your SIM card for a GigSky SIM:
- After turning your iPhone on, you may get an iMessage alert “Your carrier may charge for SMS messages used to activate iMessage”. Tap OK.
- Configure the iPhone to use the GigSky SIM: Cellular Data ON, Data Roaming ON, APN - gigsky
- Buy a data plan.
- Turn Wi-Fi off.
- Start Messages.
- Send iMessages to, and receive iMessages from, other iMessage users with an email address.
WhatsApp will work with GigSky's service, provided you have already set it up on your domestic mobile service. You will need a mobile number to set up the account, and GigSky's service does not come with a mobile number - GigSky is a data-only service.
For example, if you’re already using WhatsApp and swap out your SIM card for a GigSky SIM:
- After turning your iPhone on, configure the iPhone to use the GigSky SIM: Cellular Data ON, Data Roaming ON, APN - gigsky
- Buy a data plan.
- Turn Wi-Fi off.
- Start WhatsApp.
- You may see a “Carrier Changed” message - Tap “Keep [phone number]”, where [phone number] should be your domestic mobile number.
- You should now be able to chat and call other WhatsApp users.