My Credit Rating, According My Mobile Phone Service Provider

I recently got cut off in the middle of an international call from my mobile, with an automated message telling me I'd reached my credit limit. Hm. I checked online, and my charges to date were £19.47. £20 would make for an odd credit limit. I called. My credit limit is £50. Confirmed unbilled charges are only £19.47. Hm. Anyway, I asked to increase my credit limit. Certainly, sir, we can increase that by ... [3 minutes later] ... an additional £10. Hoo-boy! That is the max they will increase it for me, but after 30 days I can call and request another increase. "Um, ten pounds at a time?" That got a laugh out of her. She said she didn't know, but I could try back in 30 days. I suspect in 30 days I will no longer be using the Three Network for my mobile phone service.