Sometimes it can be cumbersome to always use the phonetic letters especially with a foreign registry call sign. The 7110.65 states in 2-4-20 Aircraft Identification:
b. Foreign registry. State one of the following:
1. Civil. State the aircraft type or the manufacturer's name followed by the letters/numbers of the aircraft registration, or state the letters or digits of the aircraft registration or call sign.
EXAMPLE-
"Stationair F-L-R-B."
"C-F-L-R-B."
NOTE-Letters may be spoken individually or phonetically.