Is it proper if I have sex with my girlfriend the first time we meet after being in love for months through phone?
Decisions about whether to have sex can only be made by the two of you.
Did you ever talk about it over the phone? Is it something you think she would go for?
Talk to her about it and get to know what she thinks. Normally it doesn’t matter how long or short one can take before engaging in sex for as long as both of you are ready for it and comfortable with it.
Read more here about having sex for the first time: https://lovematters.co.ke/resource/sex-first-time