Can you provide more details? Are you in the same place all day? Are you in a building where signal may be affected? Are you in a location where signal may be poor at times?
Have you rebooted your phone? Removed and replaced the SIM card?
I doubt it'll be that unless the SIM or slot on new phone was damaged somehow. If both phones use the same size SIM, they should work fine. Try putting it back in the old phone and see if it has the problem too. If it doesn't the issue could be with the new phone.