Translation of 'Missing you' depends on whether you expect to meet her/him after she/he phones you or not.

If you do, then you would go '会いたくてたまりません…近いうちに電話もらえたらうれしいですAitakute Tamarimasen...Chikai uchi ni Den-wa Moraetara Ureshi-i Des' (literally means 'I am dying to see you...would be so happy if you phone me up one of these days').

If you two can talk on the phone only but can't see each other for some reason, then you would go '会えなくて本当にさみしいです…近いうちに電話もらえたらうれしいです' (Aenakute Honto Ni Samishi-i Des...Chikai uchi ni Den-wa Moraetara Ureshi-i Des'. ('Aenakute Samishi-i' is the usual translation of 'missing you' you would find in any dictionary.)

Hope it helps.