1. Performing a trace on your sangoma card. (Assuming you have already passed steps 1-3 of Line Debugging)
Type:

#>wanpipemon -i <interface name> -c trd

If you see nothing:
Check if you are using hardware d-channel HDLC. The TDMV_DCHAN parameter should be set to 24 (T1) or 16 (E1).

If you see only outgoing packets:
Contact your telco, your line is not provisioned yet.

If you see incoming and outgoing packets:
Your card is configured properly, start a trace on Asterisk.

2. Performing a trace on Asterisk
In the Asterisk CLI, type:

#>pri show span <span number>

if it says:
pri command not found, then you did not install/configure libpri properly.

if it says:
no pri running on span <span number>
then your zaptel.conf and/or zapata.conf are not configured properly.

if it says:
provisioned, down, active
and you had incoming and outgoing packets when using wanpipemon trace, then your packets are not reaching Asterisk, recompile/reinstall zaptel (make sure zaptel modules are not loaded while doing this).