Please be aware that people who stutter usually have more trouble controlling their speech while on the telephone. Be extra patient in this situation. The person's stuttering sometimes makes it harder to understand what they are saying. If you did not understand something that was said, do not be afraid to say so. No matter how hard it is for them to say it again, this is much better than you pretending to have understood them or guessing what they said.
People sometimes wonder if it is impolite to ask questions about the person's stuttering.
This is for you to judge but, surely stuttering should not be a taboo subject. If you have a question about it, the person will probably appreciate your interest. It is in your mutual interest that it be talked about openly. Some people who stutter are, however, sensitive about it. Just follow the rules of common politeness and you should be fine.