The Doctor never answers his phone. It just one of the thing about him. Amy thinks he like the irony of living in a Phone Box and being unreachable. Rory thinks that he's lost it on some distant planet while saving the day. River's guess is more accurate. She thinks it has something to do with the past. His past.
It used to be that when his phone rang he dropped whatever he was doing to answer it. It would have been Martha or Donna or Jack calling to catch up, more often though, with some situation that needed his attention. It used to be that a phone call was a happy occasion.
Then Donna got hurt. Got hurt because of his carelessness. He had to take her memories and leave her in danger of remembering and dying. Every time he got a phone call he was afraid that it was something to do with Donna. He dreaded the shrill ringing of the mobile but he couldn't not answer it. She might have remembered. She might have died. So he always did.
Then there was the mess with the Time Lords and the Master. He'd been so angry and so scared of regeneration that he'd visited everyone. Seen how he'd changed their lives, never for the better. Tried in small ways to make amends.
Then he regenerated. He changed. He wasn't the man that Martha or Jack or Wilf or Donna or Rose knew. That was probably for the best. The man that they knew had broken them and rebuilt them wrong.
So the phone that Rose said he needed that Martha gave him that Jack and Donna and Sarah Jane had called him on was put away. It was hidden with all his memories and regrets and fears deep in the cavernous rooms of the TARDIS.
It became one of his eccentricities, never answering his phone. So the Ponds have their theories and always find new and interesting ways to contact him. They never fail.
And the Doctor never, ever answers his phone.