What is the ruling on continuing to converse with a girl over the phone with slight flirtation after agreeing to marry, and is stopping the conversation considered a defamation of the girl's reputation?

One must repent to Allah for engaging in online and phone conversations and flirtation (ghazal) between a man and a non-mahram woman. This is because it is a means to fall into what displeases Allah and is forbidden, due to His saying, the Almighty: "So do not be soft in speech [to men], lest he in whose heart is disease should covet." (Quran 33:32). The solution for one who wishes to marry is to propose to the girl through her guardian. Know that repentance involves abandoning the sin, regretting it, and intending not to return to it.