In-building GSM and 3G coverage can still be very flaky despite years of 'investment' by the mobile operators. Switching operators sometimes improves the situation but then you may not get the tariff or handset you want and can lead to coverage issues elsewhere.
I've found having a mobile handset that also supports WiFi solves this problem neatly. Increasingly I give people my Truphone number, rather than my 3 number, so I can receive high quality calls at home, irrespective of 3's flaky in-building 3G coverage. At home my Nokia E51 jumps between 3's own 3G network and their roaming partner Orange's GSM network with the result that call quality can be unpredictable. But a call on my Truphone number is consistently good quality.
Mobile VoIP operators like Truphone and DeFi that have their own numbers for inbound calls and forward to your GSM/3G number when you're outside WiFi coverage are now be a vital part of your mobile tool-kit, not just to save money but also to deliver quality of service.
Now all I need is a flat rate tariff ... DeFi has just launched Global Access which I'll be testing shortly and I'm waiting for Truphone to refresh their tariffs.