SIP registration lost

Hi everyone.

My configuration is: CentOS+Asterisk 1.8, Asterisk on public IP.
We have 3 external sip-trunks at 3 different providers.We also have 10 internal numbers.
Registration of all sip channels, both external and internal ones, fails from time to time.
It starts working again in a while, sometimes in 10-30 minutes, sometimes in several hours.

In a Asterisk logs:
WARNING[1855] chan_sip.c: Retransmission timeout reached on transmission

0ede846b163d96f958ef95937274304b@127.0.0.1 for seqno 135 (Critical Request) – See https://

wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions

NOTICE[1855] chan_sip.c: – Registration for ‘XXXXXXXXXXX@91.211.181.226’ timed out, trying again (Attempt #2)

NOTICE[1855] chan_sip.c: Peer '1001 is now UNREACHABLE! Last qualify: 27

What can cause this problem? May anything be wrong with the network or the configuration may be
wrong?

sip.conf

[general]

context=default

useragent=MyCompanyName

alwaysauthreject=yes

allowguest=no

canreinvite=no

transfer=no

base

allowguest=no

transfer=no

canreinvite=no

alwaysauthreject=yes

insecure=invite

ak-base

type=peer

host=91.211.181.226

disallow=all

allow=alaw

context=ak-out

ak-office-sip

fromuser=phonenumber1

username= phonenumber1

secret=password1

gts-base

type=peer

host=89.232.125.48

insecure=port,invite

disallow=all

allow=ulaw

allow=alaw

allow=gsm

fromdomain=89.232.125.48

context=gts-out

faxdetect=no

gts-sip

fromuser=phonenumber2

username= phonenumber2

secret=password2

[multifon]

defaultuser= phonenumber3

fromuser= phonenumber3

host=multifon.ru

fromdomain=multifon.ru

type=peer

insecure=invite

dtmfmode=inband

disallow=all

allow=alaw

allow=ulaw

allow=g729

allow=gsm

nat=no

secret=password3

context=megafon-in

friends

type=friend

host=dynamic

port=5060

qualify=yes

secret=secret

disallow=all

allow=alaw

allow=ulaw

deny=0.0.0.0/0.0.0.0

permit= public network (91.X.X.X)

permit= private network (10.X.X.X)

relaxdtmf=yes

dtmfmode=rfc2833

1001

context=outgoing

call-limit=1

nat=no

1002

context=outgoing

call-limit=1

nat=no

2 clients are behind nat.

1111

context=outgoing

nat=yes

2222

context=outgoing

nat=yes

If intermittent, network problems, or an ISP that gratuitously changes IP addresses (generally to stop customers running servers on consumer grade accounts).