Unhelpful Apple service centre - Core Group

Absolute WORST service delivery from this company. After sending my iPhone in for a routine battery replacement, they incorrectly sent me a replacement unit. I rejected this and the unit was sent back to Core Group. It's now been 3 weeks since I approved my quotation and I still do not have a repaired phone. I phoned them today and the very unhelpful staff simply told me that it "takes time". No ETR, no sense of urgency, and a total lack of commitment to get the issue resolved.