c * на Panasonic TDA-600 звонки идут, а вот в обратном направлении нет, телефонист говорит должен быть гудок при дозвоне на саму плату PRI, которая подключена к *.

inbound-pri: а у вас этот контекст имеется где-нибудь, кроме как в описании платы?

Не, не должен.
Обсудите с телефонистом два способа набора: enblock & overlap.
Найдите эти параметры в конфигурации канала ZAP.

Да, есть в extentions.confg
[inbound-pri]
include => default

а в консоль астериска что-нибудь падает?

Asterisk*CLI> core set verbose 3
Verbosity was 0 and is now 3
-- Starting simple switch on 'Zap/31-1'
-- Accepting overlap call from '1266' to '<unspecified>' on channel 0/31, span 1
[May 19 18:00:52] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '7' received on Zap/31-1, duration 0 ms
[May 19 18:00:52] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '7' on Zap/31-1
[May 19 18:00:52] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '7' on Zap/31-1
[May 19 18:00:53] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '2' received on Zap/31-1, duration 0 ms
[May 19 18:00:53] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '2' on Zap/31-1
[May 19 18:00:53] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '2' on Zap/31-1
[May 19 18:00:54] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '9' received on Zap/31-1, duration 0 ms
[May 19 18:00:54] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '9' on Zap/31-1
[May 19 18:00:54] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '9' on Zap/31-1
[May 19 18:00:54] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '9' received on Zap/31-1, duration 0 ms
[May 19 18:00:54] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '9' on Zap/31-1
[May 19 18:00:54] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '9' on Zap/31-1
[May 19 18:00:55] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '1' received on Zap/31-1, duration 0 ms
[May 19 18:00:55] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '1' on Zap/31-1
[May 19 18:00:55] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '1' on Zap/31-1
[May 19 18:00:55] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '3' received on Zap/31-1, duration 0 ms
[May 19 18:00:55] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '3' on Zap/31-1
[May 19 18:00:55] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '3' on Zap/31-1
[May 19 18:00:55] DTMF[18846]: channel.c:2116 __ast_read: DTMF end '4' received on Zap/31-1, duration 0 ms
[May 19 18:00:55] DTMF[18846]: channel.c:2168 __ast_read: DTMF end accepted without begin '4' on Zap/31-1
[May 19 18:00:55] DTMF[18846]: channel.c:2179 __ast_read: DTMF end passthrough '4' on Zap/31-1
-- Executing [7299134@inbound-pri:1] Dial("Zap/31-1", "SIP/actau/605|60|D(134)") in new stack
-- Called actau/605
-- SIP/actau-08301458 answered Zap/31-1
-- Sending DTMF '134' to the called party.
== Spawn extension (inbound-pri, 7299134, 1) exited non-zero on 'Zap/31-1'

ну так есть звонок. только уходит он от вас в транк actau

Звонки с * на АТС и обратно идут, но через определенное время прерываются, выдвая такие предупреждения в консоли:
[Jun 2 16:01:10] NOTICE[2549]: chan_zap.c:8534 pri_dchannel: PRI got event: Alarm (4) on Primary D-channel of span 1
[Jun 2 16:01:10] WARNING[2549]: chan_zap.c:2398 pri_find_dchan: No D-channels available! Using Primary channel 16 as D-channel anyway!
[Jun 2 16:01:10] WARNING[2550]: chan_zap.c:6712 handle_init_event: Detected alarm on channel 1: Yellow Alarm
......
[Jun 2 16:01:10] WARNING[2550]: chan_zap.c:1465 zt_disable_ec: Unable to disable echo cancellation on channel 30
[Jun 2 16:01:10] WARNING[2596]: chan_zap.c:3838 zt_handle_event: Detected alarm on channel 31: Yellow Alarm
[Jun 2 16:01:10] NOTICE[2549]: chan_zap.c:8534 pri_dchannel: PRI got event: No more alarm (5) on Primary D-channel of span 1
.....
[Jun 2 16:01:10] NOTICE[2550]: chan_zap.c:6690 handle_init_event: Alarm cleared on channel 31

Синхронизация рушится. D-channel отламывается, и всё что с ним завязано.
Причины могут быть самые различные: начиная с плохо обжатого кабеля, до проблем с прерыванием:
# cat /proc/interrupts

# dmesg

Заменил кабель на новый, проблема не решилась, подскажите, может настройки неверны гте-то?