iPhone 15 plus not working great

T-Mobile agreed to transfer the data from your iPhone 12 to the iPhone 15.

Apple was not involved.

I gave my old iPhone 12 away think all was well.

Apple was not involved.

Now I am stuck and T Mobile said not their problem and complain to Apple.

Apple was not involved.

What to do?

Speak to the manager at T-Mobile. If he/she can't help, speak to their boss. T-Mobile needs to take responsibility, not push the problem to a company that was not involved. In the meantime, you will likely receive some suggestions from other users here. These suggestions may assume that you are very familiar with the ins and outs with the iPhone, so you may need to request additional step by step instructions, if the initial advice that you receive is not clear.

It sounds to me like it was T Mobile who was responsible for making sure the phone was indeed in a ready-to-go condition that your required. However, that may be moot.

First check if iCloud is syncing Contacts. Of course this also assumes that your old phone was syncing Contacts to iCloud too. Check this in Settings. Read this support article about how to change your iCloud feature settings [iOS and Mac] - Change which apps sync and store data with iCloud - Apple Support

If your old phone was syncing Contacts and you have it turned on on the new one then likely something went wrong during the data transfer. Do you have any backups? At this point I believe the important thing is to get you the numbers you need to contact your appointments. Hopefully you had another record of those.

Oh, you will find most of the top helpers on this forum are "seniors" so being senior does not automatically mean somebody is not capable of understanding technology. ;-)