Flirting is showing or reciprocating sexual or romantic interest in a way that is intentionally ambiguous. It’s a way for people to gauge interest with one another without obligating either party to make any sort of direct commitment or statement of interest. Any party can choose to not reciprocate or disengage without needing to make it overt, and thus avoid embarrassment or misunderstanding.

Eventually someone has to make some sort of overture to either establish mutual interest or rejection of interest, but flirting is sort of the quantum superposition between interest and non-interest (look up ‘Schrödinger’s Date’ for the most extreme example of this).

Foreplay is the build up to actually engaging in sexual intimacy of some sort. Unlike flirting, mutual interest has already been established. More importantly, foreplay happens after, or in the process of, some form of mutual consent. Without consent it’s just assault/harassment, obviously. Depending on your relationship dynamic, flirting might be part of foreplay.

Phone sex is purely contextual. It’s either flirting or foreplay depending on whether or not it leads to sex, whatever your definition of “sex” is.