In my case, we just happen to be in a small black spot - O2 is patchy and VF non-existent. I can actually see about 10 different antenna sets too - and speaking with O2 in detail, it seems that their antenna look angles take the signal over us to get coverage to a large estate where a mast is impossible.
My employer (building now closed) used to have a GSM solution in the roof with O2, but it was never upgraded for UMTS/WCDMA, and then we switched to Vodafone and it was turned off.
I know of what you speak, as I'm pretty much next door to an EE mast, if I walk down the road I get faster speeds than indoors.
O2 here is a joke, 6 Mbps on last test, but we are in a Vodafone controlled zone, so only 10 MHz of Band 20 has been deployed. Unlike Vodafone whom have 10 MHz of Band 20, plus 10 MHz of Band 1 in residential areas, and more in shopping centre. EE trounces both of them with 40 MHz of spectrum and speeds of 150 Mbps or more.
Shame that you're in a black spot, but for corporates in-building solutions are good ideas and often managed by the operator. However femto's for home users are a pain, most networks hate them and have wanted to get rid of for years due to the pain of managing interference with the macro network.
With modern build homes and flats having metallic insulation blocking all the licensed frequency bands, WiFi calling is becoming essential.
21 years of broadband connectivity since 1999 trial - Live BQM