calls with 'no caller ID' failing / silence (they are legitimate calls)

hi i have a hospital calling me about relative, they can hear me, and can leave a voice mail, BUT i cannot hear anything from them! even the voice mail. this has happened with other no caller id calls that have been legitimate. any ideas? i am on 3 mobile UK and iphone 4s. please help as happening several times a day, all other calls in and out working fine. thanks