Samsung SGH-X426 / SGH-X427

My aunt just got this phone and since the purchase I have gotten fast busies, cust unavailable messages when I try and call her, It has been from multiple phones that I actually placed the call so I know it is either her phone or service. She forwards her business phone to her cell when she leaves her office and it is very important that most calls go through (I know, don't use cells for business, I'm in the industry but sometimes you have little choice). Has anyone else had any major problems with this particualar model compared to other GSM phones or is it probably just the general service.