Rumor has it that
According to reports, when a caller approaches a hot spot while on the phone, his or her call will transfer automatically to Wi-Fi from the carrier's regular network. The change is supposed to be instantaneous, with no noticeable difference to the caller. Of course, customers will need a dual-mode Wi-Fi/cellular handset like the Samsung SGH-T709. Though the T709 had its debut last year with T-Mobile branding, we haven't seen any further sign of the phone in months.
Though Wi-Fi voice calls could improve overall call quality, other carriers have shied away from the technology because it could allow customers to avoid using their monthly allotment of minutes. T-Mobile is tackling the issue head on by charging customers in the Seattle trial an additional $20 per month on top of their normal bills. That's pricey, yes, but it could be well worth it if you play the numbers correctly.
T-Mobile seems to have a lot in the hopper in 2007. The Wall Street Journal is speculating that HotSpot at Home will tie in with the company's planned 3G launch, which is expected this year as well. T-Mobile is the only major carrier not to offer 3G, but we know it's hard at work at something--considering it snapped up $4.2 billion in wireless spectrum at an FCC auction last year.