The number of times I've started rattling off a string phonetically and just received confused silence in return.

With how good phone connections have generally gotten I usually just spell it out normally then specify phonetically on the sound-a-like letters in the middle where enunciation and/or attention span may affect reception "Context that's C-O-N as in Nancy-T-E-X-T"

Also providing the word before spelling it helps immensely because the recipient will already have an idea of what to expect and can verify afterwords that they transcribed correctly