Zap - sip outdial

G’day all,

I have a small problem when dialing out!

details…
tdm422p
in the uk
sipgate sip acount
bt101 sip phone
pot phone plugged into tdm422p.

When I dialout out using sip account using
exten => 101,1,Dial(SIP/10000@01736888213)
(10000 is the sipgate test acount)

Using the zap

pbx*CLI> set verbose 10
Verbosity was 0 and is now 10
    -- Starting simple switch on 'Zap/1-1'
    -- Executing Dial("Zap/1-1", "SIP/10000@01736888213") in new stack
    -- Called 10000@01736888213
    -- Got SIP response 500 "I'm terribly sorry, server error occurred (1/SL)" back from 217.10.79.23
    -- SIP/01736888213-ea0d is circuit-busy
  == Everyone is busy/congested at this time (1:0/1/0)
    -- Hungup 'Zap/1-1'

Yet when I use the bt101 phone it works fine…

Any useful pointers?
I did wonder if it codecs? (not really knowing what I am talking about)

The zap phone works fine for dialing internally and dailing out over zap to a pot, the bt101 works fine as well.

[EDIT1]
I compiled and ran fxstest
On chan 3 that works

[root@pbx tools]# ./fxstest /dev/dahdi/3 stats TIP: -50.0000 Volts RING: -50.0000 Volts VBAT: -50.0000 Volts
on chan 4 which is failing

[root@pbx tools]# ./fxstest /dev/dahdi/4 stats TIP: 1.0000 Volts RING: 1.0000 Volts VBAT: 1.0000 Volts