He can’t phone her now because he doesn’t have a phone here.
⇒He doesn’t have a phone here so he can’t phone her now (so: vì vậy)
⇒He doesn’t have a phone here, therefore, he can’t phone her now (therefore: do đó)
⇒Because of not having a phone here, he can’t phone her now (because of+N/V-ing: bởi vì)