Well, recently I've switched phones with my friend (just for one day) and suddenly when I got my g1 back I couldn't send any sms!
So I called to my mobile operator and asked wth. Nice guy on line told me that he cannot help me with phone configuration because they don't know my phone model and they don't support it. What a bummer!
So I told him to give me Service center number and I configure it myself.
I never done that on this phone so I jumped to settings and started searching, searching... and nothing! No SMS Service Center number to change!
Well, that's not nice. I found two ways to do it.
First you have to dial *#*#4636#*#* and then g1 will take you to some super-cool-secret configuration screen where you can choose "Phone information" and change sms sc number.
The other way is to use app called AnyCut. It let you create shortcuts to applications and other stuff, you can create a shortcut to "Phone Info" there :)
Great thing is that you can create shortcuts to other fun things like "Debug intent", "Debug Tool" or "FieldTest" (you can also bring up FieldTest with code *#*#7262626#*#* ) which gives you some cool information about active gsm connection!
Have fun!