Based on my experience:
When I switched to TMo back in 2006, my phone shows no signal after Tmo rep inserted the SIM. He said wait a few hours, once you see "signal", that means your phone is activated. Phone was a Sony Ericsson W800. For about 1 hour, no signal. Then the signal showed up.
Fast forward to 2011. When I switched from TMo to VM, within 20 minutes of transferring the account, signal disappeared on my phone (Nokia 6301).
Of course, it could also just happen that these two particular phones "turn off" the signal if the account is not active.