Peter gave it to me so I could/can call

Sarah's mother has forbidden Sarah from seeing her boyfriend, and she's also taken her cellphone from her so she can't call him. Sarah's friend Peter gives her his cellphone, then Sarah's friend John (who knows the whole story) notices that Sarah has a new cellphone. He asks her: "Did you buy it?". Sarah replies to him:

My friend Peter gave it to me so I could/can call my boyfriend.

Can either tense be used in this case, or is only "could" correct?

Both are possible.

I'd say 'can' is a little better there, as it suggests that Sarah still has the intention and possibility of using the phone.