The fact that the stammering happens only in the beginning of the conversation, with few situation which could be putting you in anxious position shows that its just a learnt reaction to situations & not what you are. When we communicate with anyone, it's a 2 way process. It's not just with language that we communicate but also with our body, our voice tone etc. Communication is like a tennis match where you take the lead or defend yourself based on your opponent's moves. We behave differently based on the role we play. We behave in one way when situation is in our hand or other person seems to understand and like us & in another way when situation seems not so much in control. You seem to be a highly sensitive towards the way other person treats you. Telephone conversation is something which calls for spontaneous reaction for a topic or person still unknown to you. This already makes you anxious and as a reaction you stammer. Even when you ask a question over the counter, you are taking the lead without knowing how accepting the other person will be. Overall what you are going through might be the fear of being judged or put down which is being expressed through stammering. Next time when you receive a call; relax, smile and try to say as little as possible in the beginning. In a while, you will be comfortable. Over the counter, do look at the person and smile before talking. If the issue persist do not hesitate to consult a psychologist/counselor for little assistance. You will overcome stammering soon.