iPhone 3 keeps dropping signal

Ok, so I can get service for a while sometimes (it's very sporadic) while other times I constantly struggle to get service and do the "Airplane Off/On" trick to get it search for service, find it, then see how long it will hold it. I've also done the following things already:

1. Toggled Enable 3G on/off
2. Turned off phone, then waited and turned it back on
3. Did a hard reset, several times.
4. Reset all network settings.
5 Reset all settings.
6. Put a piece of tape to allow to sit more firmly against my sim card.


Not sure what else to do. I'm pretty sure it's not a hardware issue either, cause get this little trick that allows me to keep service:

*Whenever I can get service and I make a call (usually to my voicemail), it holds the service as long as I'm on the phone. But as soon as I get off the phone it quickly drops the service out. So I call my voicemail frequently since it holds service as long as I'm on a call.


Any ideas or suggestions what might cause it to keep service when I'm on a call, but be extremely sporadic when I'm not?

 

Sim card

Switched sim card to a different phone and it gets service fine. Sometimes the service works better in certain areas. Like at my house is better than at my office, but it still will drop to no service. And my home and office both get full bars normally.