When troubleshooting issues like this, it’s helpful to know that the dial tone you hear is coming from the PHONE, not the Asterisk system. (Except in the case of TDM400P analog FXS modules. There, the dial tone is generated by the card in the Asterisk box.)
Most, but not all, VOIP phones will not generate and play dial tone if they have not registered with a VOIP system, or if they have lost their registration. No-dial-tone is a really good, traditional, indication that you can’t make a call. So when you lift the handset, and it’s silent, you should go looking for a problem.
What you’re really troubleshooting is a registration problem. Click here for help with grandstream setups with Asterisk