Problem with PSTN calls (Asterisk as SIP client on embedded device)

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I’ve spent two days trying to solve this issue but to no prevail and I’m
hoping to get some help.

I’ve configured Asterisk as a SIP client, running on OpenWRT on an embedded
device with onboard FXS and ATA. Asterisk is connecting to an external SIP
provider on the Internet who in turn provides a PSTN gateway. I’m able to
make calls to other SIP accounts registered on the same server who are
outside my LAN. However, I can not make calls to any PSTN numbers. When
trying to make PSTN calls it sounds like the person at the other end is
immediately rejecting the call although I know this is not the case.

Firstly, I’m absolutely sure that the PSTN gateway is working because I can
make outbound PSTN calls with the same SIP account using other SIP clients
(Empathy-SIP, SIPDroid) from the same LAN. However, when registering the
same SIP account using Asterisk from OpenWRT all PSTN calls fail. Inbound
calls from PSTN numbers also fail while calls from other SIP clients on the
same server work fine. Thus, I’m fairly confident the problem is with my
Asterisk configuration.

The SIP accounts shows as registered in Asterisk. I’ve attached detailed
error logs. The log files ‘messages-pstn.log’ shows the failed (PSTN) call
and ‘messages-voip.log’ shows the successful (VOIP) call. Note that I have
replaced actual phone numbers and domain names with *** for anonymity.

I suspect perhaps a codec issue, but I haven’t been able to identify the
actual problem. Any ideas that will help me towards solving this problem is
greatly appreciated.