No it isn't. If you lose your phone you are in for a world of hurt, let alone all the various ways there are to intercept/redirect SMS messages and/or entire mobile accounts. To say nothing of the fact that internet access is not yet globally ubiquitous, or if you forget to pay your bill, or all these other possibilities.

A proper OTP app works offline, and more than one of them can exist for any given authentication, so you can have backups if your phone is stolen.