Only Rings Once

My girlfriend added me onto her Sprint account this past Friday. I have a new phone (Note 5), she has a Galaxy S5, about a year old. When she calls my #, she hears 1 ring only on her end, then silence until either I answer or the call goes to voice-mail. Is anyone else familiar with this issue? She & the Sprint rep yesterday thought it might have something to do with the # & changed it, but the same thing is happening. Anyone have a resolution to this? Thanks.