Think I've figured out why CellC 3G nearly unusable on Microsoft Lumia phones.

If I use something like MDMA, and force 2G and 3G, with a CellC sim card, there's like 10 dBM difference in the signal between Edge and 3G.

If I do the same on the other networks, there's virtually no difference between the signal of Edge/3G

That minor difference is causing the Lumia Windows phones to prefer Edge over 3G most of the time.