First off welcome! Depending on the provider, there could be a legitimate reason to change a phone number. However a properly reviewed and verified provider, in my experience, will rarely change their number if they can help it. Much less on a weekly basis. Everyone should have/use a hobby pre-paid burn phone, but remember.. scammers also use them as well.
Ultimately, you'll be safe if you stick to verified and reviewed providers. Starting out can be a pain, so don't go treading new territory on your own.
Stick to either well known providers or get yourself a
http://www.p411.com account. Once again welcome!
Alerts - South Texas Issues which could endanger the safety of a member or members of our community go here. Please reserve this space for legitimate alerts, and not general FYI material.
I guess mods will move this to the proper section soon enough.