If you are having problems transferring a call using an analog phones configured through an FXS channel group, modify the channel group and make the following changes.
Go to Setup > Call Routing > Channel Groups
Modify FXS Channel Group for the analog extensions
Make the following changes:
Call Waiting: YES
Transfer: YES
If you are still having problems, try Relax DTMF Recognition set to YES. Test thoroughly as this setting may make it worse