Hello maudarun,
Welcome to Microsoft Community.
I understand your need, but it's not going to happen.
This type of Surface device only accesses cellular network data with the help of a SIM card or eSIM, and even if this SIM card has free airtime, you can't use it in Surface because Surface doesn't have such a feature.
So perhaps I should advise you to use the SIM card in your phone and then buy a separate Sim card or eSIM for your Surface Pro 7+. You can check with your mobile carrier to see if there is a better deal with pure data plan without call duration, which is best for this type of device.
Best Regards,
Mitchell | Microsoft Community Support Specialist